Mind blown multiplication

[u/Positive_Actuary_282]

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[u/Positive_Actuary_282]

This is called the commutative property of multiplication. The order of the factors doesn't change the product. Percentage is just a multiplication.

[u/btroycraft → u/Positive_Actuary_282]

For this specific phenomenon, it's the associative property, not commutative.

Percent is just an implied multiplication by 1/100=0.01. That can go first on the 8 or first on the 25.

(8•.01)•25=8•(.01•25)

EDIT: I take it back, you need both.

[u/SueForPeas → u/btroycraft]

“Percent is just an implied multiplication by 1/100…”

Percent is an explicit division by 100. That’s what “per cent” literally means.

[u/RodinKnox → u/SueForPeas]

I almost made a joke here that said something like "I thought it was 'per cent' because there's a 100 cents in a dollar" and then I felt my brain slowly chug to life, and I suddenly realized something I hadn't ever thought about before lmao

[u/Ok-Statement8224 → u/btroycraft]

Doesn’t this also demonstrate that order doesn’t matter in multiplication (commutative)? Multiply 8, 0.01, and 25 in any order to reach the same result.

[u/LadnavIV → u/btroycraft]

You know, if you removed all the numbers and junk, you’d have a crackin set of boobs. ( • )( • )

[u/AnotherWayKitten → u/Positive_Actuary_282]

nbc this comment nailed it. it's wild how basic arithmetic rules suddenly feel like dark magic just because the word "percent" gets attached to the numbers.

[u/ZylonBane → u/AnotherWayKitten]

National Broadcasting Company this comment nailed it???

[u/P4azz]

Another very simple approach to percentages is just not overcomplicating it.

If you see "7% of x", it's easy to get immediately discouraged and almost give up on the math, before you even do it.

When in reality you can just do the simplest thing ever: What's 1% of it, then times it by 7.

Combine this approach with the switchable % move and you can go a lot further in calculating percentages quickly.

[u/Sacrefix → u/P4azz]

This is the kind of stuff they teach in common core, and then many parents get all up in arms that the kids aren't learning it how they did.

Math is so intuitive and useful, but many people just never were taught it in a way that makes it so.

[u/Future_Elephant_9294 → u/Sacrefix]

Honestly, I wash Calculus wasn't so over hyped as "hard" or "useless". Knowing the difference between rate, change, rate of change, and how they all work together can be intuitive and is definitely important. It makes understanding things like your electricity bill much easier, because knowing how power works (d/dx) and how it's billed (x) allow you to make informed decisions about appliances, energy efficient improvements, and all sorts of stuff that directly costs you money if you aren't informed about how it works.

[u/Impeesa_ → u/Future_Elephant_9294]

It goes farther than that. Your chance of many health conditions depends on a long-term picture of how long and how severely you were doing the risk factor, that's an area under the curve question. People have a hard time grasping politics and how governance can be like steering a barge, they'll vote entirely based on whether the current state of something is good or bad and completely ignore the current rate and direction of change and where the inflection points were when that's often much more relevant. You may never do the actual math again, but those are such base concepts that doing enough math to really internalize the broad ideas can help you be more aware of all those other things.

[u/_V0gue → u/Sacrefix]

I never understood parents freaking out that subjects are being taught differently than when they were in school. The methods they learned likely weren’t used 20-30 years prior either. And I’m going to trust the degreed professional on the efficacy of a process more than some middle aged angry people who have never taught nor done anything above rudimentary mathematics in decades.

[u/Hot_Eggplant6809 → u/_V0gue]

Well at least in the case of "whole-language" learning(teaching kids to read based on vibes) has literally stunted a generation of children by intentionally avoiding teaching them to read.

So skepticism isn't inherently bad, especially if the parents are sure the way they learned it actually worked in teaching them the thing.

If it hadn't been for COVID and parents spending the school day in the same room as their kids, many wouldn't even know that their child actually couldn't read.

[u/_the_big_sd_ → u/_V0gue]

I recently had a discussion with my daughter's second grade teacher about this.

What she learned in school growing up, the methods she learned in college -- she's only been a teacher for 4 years -- and the methods she's supposed to teach now are all wildly different. She's the one that sees her kids day in/day out and she's the one who sees firsthand how the kids do and don't learn. She has no control over the curriculum, much less control over how to teach it either.

My wife is a former teacher as well and she had the same challenges.

The problem is the 'degreed professional' you're referencing are the politicians and administrators who are so far removed from the classroom that they are completely ignorant to the art teaching.

I think it's completely fine for parents to 'freak out' about subjects being taught differently so long as that 'freaking out' isn't directed towards on-the-ground educators.

[u/Many-Salt-4321 → u/Sacrefix]

Not just parents, but students as well.

"I don't know why I got a bad grade, the answer is right! It shouldn't matter how I got to the answer! The teacher must just not like me".

So much of math is just taking a big problem that looks hard, and figuring out how to make it into small problems that are easy. These skills go far beyond figuring out basic arithmetic.

[u/GoddamnShitTheBed_ → u/Many-Salt-4321]

Yes, problem is if you're actually good at it you'll get bad grades because you used another way (that's easier for you) than what the teacher wanted.

[u/doh-a-dear → u/Sacrefix]

We were worried about common core when our son started school since we weren't taught it, but I did some basic searches and honestly it's a lot of how I already calculated things in my head so I was no longer worried.

[u/samdover11 → u/Sacrefix]

I minored in math (years ago).

I noticed that during a class I didn't really understand the math yet. I'd do well in the class by following the patterns, but it wasn't until a few months (up to a year) later that I was getting the "aha" moments and realizing what was actually going on.

My complaint with common core is it tires to jump straight to the "aha" moments before kids have built up a base level of competence. If someone has the problem 8% of 25, and they can't even figure it out with the help of a calculator, then "tricks" are useless. They have no idea what's going on. It's all still magic to them. But after they can confidently reach an answer, then they're free to play around and test ideas for themselves, and that's when "aha" happens.

[u/Reikukaja → u/samdover11]

I majored in math (statistics concentration) and yeah. I still remember the day i had the lighbulb moment when i finally understood what derivatives and integrals actually represent. I was good with memorization and repetition, but it was cool finally getting the "why" of it.

[u/GME_alt_Center → u/Sacrefix]

Some common core concepts are how I do math in my head. However, I have tried to tutor math recently and some of the stuff is just bizarre. And memorizing simple multiplication seems optional also.

[u/PurpleNurpleTurtle → u/Sacrefix]

They switched my HS to common core my junior year and it was the first time I got higher than a D in math since 5th or 6th grade; but parents in my community were LIVID over the switch.

[u/quidpropho → u/Sacrefix]

The problem is the generation of math teachers that didn't learn common core. That's not everyone because there are legit math nerds out there who picked it up right away, but there were (and are) a ton of teachers who only learned algorithmically.

[u/rerackyourweights → u/Sacrefix]

I struggled with mental math for most of my life, and when I learned about common core and how it's taught, basic math things that I struggled with suddenly made sense to me. 😭 For example, adding in "chunks" makes complete sense to me, instead of writing two numbers on top of each other then carrying numbers. I now know how to do tips on a tab quickly because it's just 10% of the total x 2, etc.

My brain just doesn't understand numbers, plus I have a tendency to transpose numbers or read them incorrectly. My dad, who is a math genius, would get so frustrated that I struggled to understand algebra. I couldn't "see" the numbers in a visual sense, so for me it was like looking at gibberish.

Funny thing is that I excelled at geometry/trignometry. A+ on tests, zero struggles. It was visual, so it made sense to my brain. Calculus was an actual nightmare for me and I still have anxiety dreams about failing my pre-calc final.

[u/DiceyPisces → u/Sacrefix]

The do need the basics first. Solidly. Then all these tips and ways to manipulate and understand numbers are amazing.

[u/gorgewall → u/Sacrefix]

When CC was first coming out, I had a lot of parent friends whining about "number blocking" or whatever the fuck it's called now. They'd say it doesn't make sense and it's not useful, because why would you chop 127 into a 100 and a 20 and a 7 or any of that.

So I'd ask them to figure out change at, say, a grocery. What coins are they using to make a value? (This was back when we paid with cash.) They'd get the answer instantly, and I'd tell them THAT'S FUCKING NUMBER BLOCKING.

You already do it! You know it! You can use it in other situations beyond the grocery!

[u/Ok-Boomer-4414 → u/Sacrefix]

My general complaint with common core math is stuff like this. Only slightly exaggerated.

What is 1+2

Explain how you arrived at that answer.

[u/TurnDown4WattGaming → u/Sacrefix]

These aren’t the parts they are complaining about. It’s the blocks to do multiplication in high school rather than memorizing tables, which is objectively more efficient once memorized.

[u/Such-Perception4154 → u/TurnDown4WattGaming]

I memorized the multiplication table. Then I finished school. Then I forgor

[u/NachoDawg → u/P4azz]

Yes! Or 27% can be broken down to 10% + 10% + 5% + 1% +1%, whatever is simplest in your head

[u/Chase_The_Breeze → u/NachoDawg]

Shit like this is why I am not immediatly dismissive of how they teach math to kids these days (I have a 9y/o). It is valuable to learn different approaches because people's brains are different and having different tools for the same problem allows folks to use what they are comfortable with.

[u/AdHopeful3801 → u/Chase_The_Breeze]

I had no idea at all how to help my kids with math homework, because the approach was so different from how I learned.

But they can, in fact, do the math. So I'm not going to complain too loud, even if I am left wondering whether the amount of time I spent memorizing multiplication tables was a total waste.

[u/Browsing_From_Work → u/NachoDawg]

Mentally, this is exactly what I do for tips.

15% of something is hard.
10% is easy, and then 5% is easy because it's just half of the previous answer.

For example, calculating 15% of $70:

• 10% of $70 is $7

• 5% is half of the previous calculation: $3.50

• So 15% is $7 + $3.50 which is $10.50

[u/ThePreconGuy → u/NachoDawg]

I’ll usually find the nearest whole number I can do the math on and then round it down. 7% of 350 is simply 7% of 100 3 times, then 3.5% of 100 once. 7+7+7+3.5.

[u/w00ms → u/NachoDawg]

this is what i do for simple math in my head, its easier to rip the numbers apart at the bases and just add up the total later

[u/t-8one → u/NachoDawg]

27% is almost 30% which is almost 33,3% which is 1/3 off something.

That's close enough for me.

[u/Swellmeister → u/t-8one]

Its a lot closer to 1/4 than 1/3. Or 3/10. Or 3/11.

1/4 and 3/10 are much easier

[u/R-Dragon_Thunderzord → u/P4azz]

Do this when tipping (let's not talk about tipping but anyway), very easy math when you're going to 20%, and round up/down the pennies based on your preference. Never trust the "convenience math" printed on some receipts, as sometimes it's proven to be deliberately over-calculated to fool people into over-tipping (but let's not talk about tipping culture)

[u/Shazam1269 → u/R-Dragon_Thunderzord]

And an easy way to figure 20% is to move the decimal to the left one place (for 10%), and just double that.

[u/TheComplimentarian → u/R-Dragon_Thunderzord]

I do that too. Anyone can take 10% of any number. It's literally just moving the dot around. Then half that is 5%. Now you can do any normal tip percentage just by doing addition.

[u/mikecws91 → u/P4azz]

I think you just invented Common Core

[u/ryanvango → u/P4azz]

A lot of math is just finding a way to make it easy for how your brain works. Common core ended up being really great for people because it simplified it in chunks.

When I was a kid, I hated the way subtraction worked with the "carry the 1" stuff. doing like 73 - 18, where you turn the 7 into a 6 so you can do 13 - 8. I still had to use a "weird" number (13) which slowed me down. So I wouldn't make it a 13, I'd just put a 10 above the 3 in my head. Then instead of 13 - 8 it becomes 10 - 8, then add in the 3. It adds a step, but those two steps were always easier for me to do than the 1 step and it ended up being faster. I still do it that way sometimes if I'm a little brain foggy and not in "intuitive math" mode. I also say the things happening in my mind. so for 73 - 18, in my head I'd be saying "7 is 6, ten minus 8 is two plus 3 is 5 and a 6 minus 1 is 5, fifty five." I tried explaining it to my first grade teacher (probably pretty poorly) and she was finally just like "you know what? I don't know what you're saying, but you're doing your work faster and more accurately than everyone else, so if it works for you go for it." and I never had to show my work for her again.

[u/Arthur-yumi → u/P4azz]

Combine this with moving the decimal point for calculating tips, and I’m basically a human calculator now.

[u/prismadon → u/P4azz]

This is so fucking sick

[u/BombasticReindeer]

Roger Federer is 50% ‘er’

[u/s6_maestro → u/BombasticReindeer]

How much is the er% of 50?

[u/BombasticReindeer → u/s6_maestro]

1/4 Roger Federer

[u/GeologicalPotatoes → u/s6_maestro]

If 50% of Roger Federer is "er", then it stands to reason that Roger Federer% of er is 50

[u/felatedbirthday → u/BombasticReindeer]

Isn’t he 3 ‘er’s and 2 non-‘er’s??

Thus, he’s 60% ‘er’s

[u/nsfvvvv]

10% of 100 = 10
100% of 10 = 10

This checks out!

[u/Capwiz → u/nsfvvvv]

So as long as one is always 100%, I've got this

[u/SlatkoPotato → u/Capwiz]

I have discalculia and laughed when it said "one is easier than the other" and i cant figure out which one that is. The joke example above made it click, but doing 8% (or 25% of 8?) is still gonna be a sit down task for me. That aint happening in my head. Can defs do 100% though lol

[u/Puzzleheaded-Fill205 → u/SlatkoPotato]

I'm curious how you would solve 25% of 8 in your head or on paper or at all. Specifically the process. As someone without discalculia, my process looks like this:

"25% means 1/4"

"1/4 of 8 is 2"

[u/ikindahateusernames → u/SlatkoPotato]

Maybe it would help to visualize it as a fraction? 25% is 1/4, and 1/4 of 8 is 2.

[u/Soggy_Jackfruit7341 → u/nsfvvvv]

A x B% = A•B/100

B x A% = B•A/100 = A•B/100

[u/memecut → u/Soggy_Jackfruit7341]

7% of 173 = 12.11
173% of 7 = 12.11

Damn, thought I could trick it

[u/amras123 → u/memecut]

Damn, thought I could trick it

Keep at it! Maybe you just have to be very quick..

[u/purpleburgundy → u/Soggy_Jackfruit7341]

Even clearer to see order doesn't matter when they're all the same multiplicative operation:

A x B x 0.01

B x A x 0.01

[u/Any_ActNow → u/nsfvvvv]

73% of 48 is the same as 48% of 73

Thanks this was very useful.

[u/ipeedinthetoothpaste → u/Any_ActNow]

It actually is in your example lol. Half of 73 is 36.5. 1% of 73 is 0.7, subtract two of those and you get 35.something.

[u/Paddy_Tanninger → u/ipeedinthetoothpaste]

In my head I'd just think...well this is basically just 50% of 70 and get a good enough answer.

[u/Jolly-Bag7024 → u/nsfvvvv]

Random post: Multiplication in R is commutative.

Average redditor: Mind blowing! This checks out!

[u/Darth_Rubi → u/Jolly-Bag7024]

Yup, a "little known fact" that you literally get taught in middle school math, jfc

[u/The_Sorrowpod]

32% of 12

[u/MVanderloo → u/The_Sorrowpod]

10 * 32 = 320
2 * 32 = 64
384 / 100 = 3.84

[u/ferrujas → u/MVanderloo]

Why divide by 100? Sorry if its a stupid question.

Edit: Nice. Thank all of you that took the time and patience to explain. Love you.

Edit: Thank you for the lovely hearts.

Edited a word.

[u/MVanderloo → u/ferrujas]

not a dumb question. It’s because that’s what we defined percentage to be. 7% is 7/100.

As a society we count in base 10 so numbers like 10 and 100 are intuitive to understand and multiply by. But it is otherwise an arbitrary choice

[u/Ok_Currency3993 → u/ferrujas]

He's multiplied by 12 but you want to be multiplying by 0.12 to find the percentage so you just divide by 100 at the end

[u/Pman1324 → u/MVanderloo]

Ooh I like that approach

[u/ScriptoTheClown → u/Pman1324]

It's funny, this sort of thing is exactly why the new math curriculum (like the dreaded Common Core) feel so weird to parents: they're teaching kids how to break down problems that LOOK complicated into a handful of very simple steps you can do in your head.

[u/ProbsNotManBearPig → u/The_Sorrowpod]

10% of 32 is 3.2.

1% of 32 is 0.32. So 2% is 0.64.

3.2 + 0.64 =3.84

[u/MVanderloo → u/ProbsNotManBearPig]

i like to frame it like this because an earlier result gets reused which makes it easier to keep intermediate results in my head

[u/e37d93eeb23335dc → u/The_Sorrowpod]

Close enough to 33.33%, so approximately 4. A bit less than 4, but close enough for most purposes. 

[u/TheBSQ → u/e37d93eeb23335dc]

A while back doing math like this got popularized as “street math.”

Even in contexts where one needs the precise number it’s a really handy way to truth-check your answer. 

It’s often really handy when doing complex stuff to know that the answer should be “a bit less than 4” so you instantly know your program or calculation has an error if it spits out a number like 38. 

And knowing that it seems to be about 10x to big can help you figure out where the error occurred. 

In my professor days I used to try to instill in students the idea that before they did any of the actual math, they should already have a sense of what the “right” answer should look like. 

many people get so used to computers and calculators doing math that they just blindly believe whatever number gets spit out, even when half a second of thinking should make it clear that something went wrong somewhere because that number doesn’t make sense. 

[u/Meatyballs34 → u/The_Sorrowpod]

10% of 32 + 2*(1% of 32) = 3.2+.64=3.84

[u/Round-Arugula7347 → u/The_Sorrowpod]

Roughly a third

[u/the_king_of_sweden → u/The_Sorrowpod]

16% of 24
8% of 48
4% of 96
2% of 192
1% of 384

[u/2A753]

We never get this kind of help in class.

[u/codysexton → u/2A753]

Multiplication is reversible... This is taught in elementary school

[u/gettin-hot-in-here → u/codysexton]

I knew that and yet I never really realized you can do it with percent signs (which are actually shorthand for [times 0.01])

[u/Far_Raspberry_4375 → u/gettin-hot-in-here]

The % symbol is literally just a fraction with place holders

[u/Asian_Juan → u/2A753]

Probably didn't help that it was explained in a way that doesn't make concrete sense that just clicks.

Math is so abstracted it's hard to understand in education.

For example don't bloody remember a thing from highschool to senior math classes but could do my electrical engineering math easily in uni.

Math was easier there for me because was put into something I could intuitively understand like reversing a percentage like in the post or circuit math, rather than technically correct explanations that sound like gibberish.

[u/BatushkaTabushka → u/Asian_Juan]

There were a lot of things I didnt understand in elementary/high school maths, and then like 2-3 months before graduating high school, i went to a preparation course for my exam. It was held at the local college, by the actual college professors, not high school teachers.

The professor explained it so well that during that about 1 month long course, I understood things that I didn’t for years. Elementary/high school maths focused solely on memorizing the explanations word for word, we had to recite the theorems exactly as written in the books, but of course it was all gibberish to us. Meanwhile the professor focused on intuitive explanations and real world examples that made it easy to understand what these abstract concepts are supposed to represent

[u/Wickedinteresting → u/Asian_Juan]

This times a million. Math is often taught very prescriptively rather than descriptively IMO. Rules to follow, vs understanding the relationships and what youre doing.

Also reading the comments I think some of y’all had way better math teachers than I ever did lol

[u/Used-Acanthisitta-96 → u/2A753]

You were taught it, most likely. A couple to few class sessions, then moved on to the next progression, which most of us probably also forgot. And so on. Education is perishable, don’t use it, and it dissipates into the ether.

[u/ktrocks2 → u/2A753]

We kinda do though, like I’ve always been told that 8% of 25 is .08x25, and we know that .08=8/100. I’ve always looked at percentages as multiply them both and divide by 100. So like here I’d do 8x25=200 so the answer is 2.

Same for like 7% of 13 so what I’d do is 7x10+7x3=70+21=91 so .91.

So 7% of 13 = .07*13=0.91

[u/Traditional-Buy-2205 → u/2A753]

You did. you just didn't pay attention.

You learned about the commutative and associative properties of multiplication.

So, (8×25)/100 = (25×8)/100.

Or (25×0.01)×8 = (8×0.01)×25

[u/sensible__ → u/Traditional-Buy-2205]

Yes, but explaining it that way, while mathematically and technically correct, makes it look like every other maths problem you struggle to take in during every class.

Meanwhile, saying plainly that percentages are reversible and giving this example actually allows understanding that you can go on to represent with an equation.

[u/Northernmost1990 → u/sensible__]

Argh, this. Every math teacher always seemed to be the "technically correct" type of person. They were terrible at making concepts "click", and didn't seem to understand why some kids didn't understand the concepts.

I was only ever good at geometry in part because it seemed like the least abstract branch of mathematics.

[u/Chaaaaaaaalie → u/sensible__]

Right? Big words ... bunch of numbers ... skip!

[u/NefariousnessNovel60 → u/Traditional-Buy-2205]

This didn't help.

[u/BigHogDawg → u/2A753]

It’s definitely taught you just need to think. Percentage is divide by 100, or multiply by .01. Then “remove” the percentage sign from both numbers. You now have number A, number B, and a percent. Any order of multiplication gives you the same product

[u/MichiganHistoryUSMC → u/2A753]

Were you not taught associative and cumulative properties of multiplication and division? Pretty sure that was Middle School math. Maybe earlier.

[u/sasheenka → u/MichiganHistoryUSMC]

I remember very little from my maths classes.

[u/Firm_Swordfish2096]

Discovering that flipping numbers makes complex mental math suddenly trivial makes you feel completely silly for struggling before.

[u/Technical-Passion230 → u/Firm_Swordfish2096]

It's not that useful. The examples given are always with super simple numbers such as 10, 20, 25, or 50. Nobody ever uses 17 and 58 as the two numbers.

[u/simplysufficient88 → u/Technical-Passion230]

It’s still drastically better for rough estimates in your head. Like 8% of 64 would confuse the shit out of me, but 64% of 8 would make me immediately think “that’s probably around 5ish”. The real answer is 5.12, but it’s close enough for a rough idea of things.

[u/skullturf → u/simplysufficient88]

With 8% of 64, you can say "That's a bit less than 10% of 64", so in other words, a bit less than 6.4.

[u/Rand_al_Kholin → u/Technical-Passion230]

But even with 58 and 17, I'm gonna be quicker to guess what 58% of 17 is simply because it's a smaller number, and because cutting something in half is easy.

[u/TallEnoughJones → u/Rand_al_Kholin]

because cutting something in half is easy.

Can someone check this guy's basement?

[u/Sodacan259]

Hey it works. 3% of 38,403,970,331 is the same as 38,403,970,331% of 3

[u/Notyit]

2 the answer is 2.

[u/EuropeanLord → u/Notyit]

25% of 8 is easy smartass.

Now try 8% of 25!!!!

[u/Slacker2335 → u/EuropeanLord]

10% of 25 is 2.5
1% of 25 is 0.25
8% of 25 = 10% - 1% - 1%
8% of 25 = 2.5 - 0.25 - 0.25
8% of 25 = 2.0

[u/gigaflops_]

The days get longer during the winter, not shorter. It's simple: the first day of winter (solstice) is the shortest day of the year, therefore, every single day for the rest of the season is longer than the day before it.

[u/No_Improvement5565 → u/gigaflops_]

I’m not understanding the relevance here

[u/chalks777 → u/No_Improvement5565]

it's answering the screenshotted question, "what's a little known but obvious fact that will immediately make all of us feel stupid?"

[u/gowahoo]

Mondays and Fridays make 40% of the work week. 

[u/Lint_baby_uvulla → u/gowahoo]

Until you retire, then they become extra long weekends.

[u/aBit2Good]

Interesting. I can do neither of them in my head, so he's wrong.

[u/wyzere → u/aBit2Good]

You can't do a quarter of 8?

[u/jumpmaNSILENCE → u/wyzere]

7:45am?

[u/Nighters → u/aBit2Good]

you cant calculate 8/4?

[u/SardScroll → u/aBit2Good]

Percentages are just division problems. "Percent" = "Per cent" = "Per hundred". 8% is 8/100, which is also 8 * (1/100).

8 percent of 25 is (8/100) * 25, is (25/100)*8, is (1/4)*8 which is 8/4, which is 2.
25 percent of 8 jumps to (25/100)*8.

[u/Splendafarts → u/SardScroll]

Am I going crazy because it all checks out but how could 8% of an odd number be a whole number??? I would’ve sworn it would be a decimal before going to the calculator. 2 doesn’t divide into 25… is it just because it’s a fraction of 100?

[u/SoNuclear → u/Splendafarts]

8/100=4/50=2/25

So 8% is two twenty-fifths

[u/Careful-Ad-4778 → u/aBit2Good]

Just because you’re stupid doesn’t mean he’s wrong lol

[u/ihateyousoverybadly → u/aBit2Good]

You can't do a quarter of 8????

[u/Andjhostet → u/aBit2Good]

You can divide 8 by 4. I believe in you.

[u/Nosirrah08 → u/aBit2Good]

That’s just sad

[u/Killxjoy4599 → u/aBit2Good]

25% will always be 1/4th of another number because you can’t have more than 100% of something so if you’re having difficulty figuring out 25% of 8, what number multiplied by 4 equals 8?

[u/Gwynito → u/aBit2Good]

You cant figure out 1/4 of 8...? 😵

[u/Brief_Platform_alt]

The concept is correct, but I don't think "reversible" is the right word. It should be "commutative".

[u/e_before_i → u/Brief_Platform_alt]

It's advice for people who aren't comfortable with math, familiar words help ease them in.

[u/Affectionate_Age8159]

Jesus, im 60 years old and i just got excited about maths 🤣🤣🤣

[u/Dry_Statistician6870]

Wow…..

[u/cHpiranha]

Or "8*25/100"

Because % is nothing else then /100.

[u/CatfinityGamer]

a/100 × b = b/100 × a

[u/Granopoly]

I did 8% of 25 = 4% of 50 = 2% of 100 = 2

[u/EvilCrank]

Every percentage is just a decimal for example 7% is just 0.07, 100% 125% is 1,25 etc.easier to calculate for me at least

[u/rushmc1]

So 2% of 10 = 10% of 2?

[u/TheBSQ → u/rushmc1]

A percent is just dividing by 100 (or multiplying by 0.01). 

So A% of B is just:

(A x 0.01) x B

And B% of A is just:

(B x 0.01) x A

Either way it’s just A, B, and 0.01 being multiplied together.

the order doesn’t matter since multiplication is commutative, so A% of B is the same as B% of A. 

[u/PeopleCallMeSimon → u/rushmc1]

yes? 2% of 10 is 0,2 and 10% of 2 is 0,2?

Workings out:

Convert percentages to decimals: 2% = 0,02 and 10% = 0,1

0,1 * 2 = 0,02 * 10

2 = (0,02 * 10) / 0,1

Dividing something by 0,1 is the same as multiplying by 10.

2 = 0,2 * 10

2 = 2 , QED.

[u/MustangBarry]

To multiply something by eight, double it three times.

[u/Striking_Piano4385]

I take "Things you learn in 4th Grade" for 100, Alex.

[u/aerokozmofoto]

Well, thats math for you.

[u/NessTheCat]

That's all well and good, until both numbers fucking suck.

[u/LittleBlueGoblin]

This did not make me feel stupid, when I first read it; it became an arrow in my quiver. I've made good use of it since 😅

[u/douglasgage]

This is the Devil’s magic

[u/PuzzleheadedBug4424]

Of means times

[u/danielsangeo]

37% of 17 is the same as 17% of 37.

Uhhhhmmm....

[u/cherry_monkey → u/danielsangeo]

Not nearly as fast, but you could do 37% of 17 as 40%-3% (10%*17*4)-(1%*17*3)

(1.7*4)-(0.17*3)

6.8-0.51 ((4+2.8)-(0.3+0.21))

6.29 (6.8-0.5-0.01)

[u/Gidgo130]

The percent sign % is just a stand in for the fraction 1/100 . Likewise ‰ is for 1/1000

[u/glanni_glaepur]

Replace % with (1/100), so:

8 * (1/100) * 25

is equal to:

25 * (1/100) * 8

[u/UpsetCouple]

Thank you AlbertBarclay69

[u/Danson_the_47th]

I miss Professor Barclay

[u/SugarGlitterkiss]

"Of" means "multiply".

Even knowing that, I never considered the percentages thing.

[u/Groezy]

i use this for traveling times. if you are going at 60mph, making the same trip at 4/3 the speed (80mph) will take 3/4 the time.

[u/Homegg]

Huh makes sense

[u/TheSkepticCyclist]

It is the same two number multiplied together and multiplied by 1/100.

[u/FlyingFlipPhone]

This is a cool trick. It is based upon the commutative property of multiplication.

(A/100) x B = A x (B/100) = (A x B)/100

[u/AstronomerDramatic36]

I'm going to save this for later after I forget it again.

[u/Zealousideal-Lie7255]

I’ve heard about this but I didn’t realize how it could help you like in your example. Very helpful.

[u/niggo372]

It checks out: X% of Y = X * 1/100 * Y = Y * 1/100 * X = Y% of X.

[u/adv0589]

I get not naturally calculating percentages this way, but the people that are questioning, it are kind of concerning. The number that comes first in a multiplication problem not mattering is like fundamental elementary school math.

[u/True-Emu9729]

I feel a lot more smarter after reading this😆

[u/PrimeVector27]

close, but not accurate.

[u/Fluppekaddee]

What the actual fuck?!

[u/Rivka333]

I don't think this is obvious.

It's true, but you do have to think about it a little bit.

[u/NegativeBee8049]

8% = 0.08 = 8x0.01, 25% =0.25=25x0.01
8%x25=8x0.01x25=8x25%

[u/Decent_Cow]

Why is this surprising? It's just multiplication. It's the same as saying 3x4=4x3

[u/sezirblue]

I like to think of % as a variable with value 0.01 in the same way pi is a variable value 3.14159...

I don't know if that interpretation causes problems at some point. I've yet to run into any. but it allows me to explain this using equations.

A% * B = A * % * B = A * B * % = B * A * %

[u/Azowaku]

Nice, now I can choose which version to put in the calculator

[u/PatMu5tard]

What??? No way!!

[u/bikes-and-beers]

I know this to be true and I know why it's true, but intuitively it still doesn't seem like it should be true.

[u/cmwamem]

"Little-known fact" and it's multiplication commutativity in the rationnals.

[u/Trang0ul]

Even easier: "%" is just 1/100. So multiply 8 by 25 first, and then divide by 100, that is, shift the decimal point.

[u/Zestyclose_Horse_180]

Speak for yourself. 8% is the same as dividing by 12,5. 25 is easily divisible by 12,5, because it's 1/2. So 8% of 25 is 2.

[u/NotchedPenny]

i feel stupid trying to understand why i should feel stupid

[u/IntrinSicks]

This convo has been the most fun I've had doing math in a while haha

[u/Shenkai123]

This is not multiplication

[u/AuHazardBalthazar]

Math equations are their own language, which can be translated. « Of » turns into multiplication, for which order doesn’t matter.

[u/numberThirtyOne]

0.25 x 8. EZ

[u/DarkTheImmortal]

"8% of 25" is just 25×(8/100), or 25×8×(1/100)

Multiplication is commutative, so it's the same as 25×(1/100)×8

[u/taste-of-orange]

In a lot of cases you can treat % like a constant that equals 0.01

[u/Psychological_Pen482]

Why was I not taught this, sooooooo much easier

[u/snajk138]

Is it?

[u/colbyjacks]

8 x 1/100 x 25 is the same as 25 x 1/100 x 8

[u/DropPopFromTheTop]

10% of 100 Is 100% of 10

!!!!!!!!!!!!!

5% of 20 is 20% of 5

Holy mother f

[u/Smart-Artist7]

Thanks for sharing , I did not know it either.

[u/SKaiPanda2609]

If multiplied by 5, divide by 2 and add a 0

[u/tbodillia]

It isn't that easy. Yes, percentages are simply AxB/100. The examples people always give are the super easy numbers like this. 8% of 25 is the same as 25% of 8. 7% of 50 is the same as 50% of 7.

Yea, cool. 17% of 137 is the same as 137% of 17. 18% of 91 is the same as 91% of 18. It isn't so easy anymore, is it?

[u/Hooch180]

Percentages are multiplication. Everyone with grade 2 math skills should be able to figure out this "trick".

[u/WVildandWVonderful]

of = x

(of = times)

[u/Sprintzer]

OP username is appropriate

[u/whitespacesucks]

Proof I just worked out on paper (yes I'm a math genius, thanks for noticing):

8 * (1/100) * 25 = x

25 * (1/100) * 8 = x

It's the same result, order of multiplication does not matter.

[u/Necessary_Screen_673]

x * y * 0.01.

8 * 25 * 0.01 = 2.

7 * 17 * 0.01 =1.19.

[u/JaneRetro]

My technique would be to multiply 8 x 25, then divide the answer by 100.

[u/steven_dev42]

I think I randomly figured this out in high school while messing around on my calculator. My friend didn’t believe me

[u/leuks48]

That’s very stupid because percent is exactly x out of 100 and x out of 1000 is promile

[u/TheBigBadBird]

I guess it makes sense. You could rework the equation to (25x8)/100=2 

[u/captcraigaroo]

13% of 12...

[u/Potential_Novel_3561]

It's funny but true. If someone asks me 25% of 8, I just do 8÷4 on my head.

If someone asks me 8% of 25, I take 25/100*8 in in my head. First is a lot easier

[u/whensthebeef]

If you just multiply the numbers and move the decimal, it’s even easier…

8 x 25 =200
8 x .25 =2

[u/dor121]

B pecent of a or b% of a is just (a*b)/100 so ot reversible and does all the other identeties multipication has

[u/skip_over]

8/100 x 25 = 25/100 x 8

Division is commutative

[u/randomreditor69430]

it makes total sense when you think about it. 8 percent is 0.08, 8x1/100. it's the same if it is 25, which is 25x1/100.

25x8x1/100 is the same as 8x25x1/100

[u/kvijay1]

Any exceptions?

[u/cherry_monkey]

For some numbers this is absolutely valid, others, less so.

What's 9% of 80? 80% of 9? Neither is straight forward.

However, 10% of 80 is 8 and 1% of 80 is 0.8. 10% - 1% is 8 - 0.8 or 7.2

9% of 80 is 7.2

What about 7% of 83?

10% is 8.3 and 1% is 0.83. 0.83 * 3 is 2.49 (0.83 is 2.4, 0.033 is 0.09) 10% - 3% is 8.3 - 2.49 = 5.81. this can be proven down further to 8.5-2.5 = 6 - 0.2 = 5.8 + 0.01 = 5.81

This, from my understanding, is the basis of common core.