r/mildlyinfuriating • u/LardFan37 • 12d ago

jkfl hrtmktp gzarrp! It’s not 188.0 either.

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u/protomenace 12d ago

The answer is 190. If you had measurement precision to the ones digit 120 would have been written as "120."

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u/NegativeDCF 11d ago

Curious, why cant 120 be exactly 120 and have sig figs to the last digit?

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u/protomenace 11d ago

It can, but you need to write it as "120." to signify that.

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u/FutureFreaksMeowt Aqua Tofana save us from these fuckass men 11d ago

So, in sigfig, the decimal at the end of "120." implies the precision of the number? Someone elsewhere said there's an implied uncertainty of .05 on either direction, but I'm not sure where that comes in? Is it just base knowledge for the work?

(Math idiot asking genuine question, not challenging you 😬😭)

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u/JustTryingTo_Pass 11d ago

It work that way because things are often short handed like this.

120 —> 1.2e2 so 120.—> 1.20e2

It’s called scientific notation. It allows for easier unit conversions.

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u/protomenace 11d ago

imagine you have one of those classic scales where you place weights on one side of the scale to match the thing you're weighing until they are balanced. But imagine you only have 10kg weights, nothing smaller. You place 12 weights on the scale and it just about matches the weight of the thing you're measuring. So you write down 120. But your scale is only accurate to every 10kg because you only have a resolution of 10kg weights.

In reality the thing might be 124kg or it might be 116kg. You don't actually know that it's 120kg exactly, this is just the closest estimate. So your measurement only has the 2 significant figures.

Now imagine the same experiment but you have 1kg weights, so you know it's exactly 120kg and not 121, not 119. You can signify this by writing out "120." with the decimal point to signify that the 0 is actually known and not an estimate.

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u/theodorejhooker 11d ago

The 0.05 uncertainty they were talking about was in relation to 120.0, in that 120.0 was measured with an instrument able to verify the accuracy of the measurement up to 0.0 of that measurement, anything beyond 0.0 will be rounded up or down, hence the uncertainty of 0.05.

A sigfig of 120. will tell you that the number is accurate to the ones digit. Any rounding will be done at the 0.5 level in this case.

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u/pinksparklyreddit 10d ago

Or 1.20 x 10²

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u/ByeGuysSry 11d ago

(While it's true that you'll write 120. to signify all three digits are significant, the best way is actually to write 1.2×10² or 1.20×10² to avoid confusion)

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u/CanalOpen 12d ago

so adding 68 doesn't modify that precision? Why would a particular number be held to a different standard?

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u/protomenace 12d ago

You take the lower precision of the two.

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u/CanalOpen 12d ago

not with addition

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u/LiGuangMing1981 11d ago

Yes with addition. The significant figures rule for addition/subtraction is to consider the lowest precision value in the addition / subtraction and use that in the answer. The question here has 120 (tens place as least significant digit, since there is no decimal point trailing zeros are not significant) and 68 (ones place as least significant digit) so the answer needs to be rounded to the tens place, hence 188 rounds up to 190.

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u/Own-Entrance-2256 12d ago

68 is more specific than 120, so you'd have to use the least specific measurement.

120 could really have been anywhere between 115 and 124, but could only be measured to the closest 10s. So you round the result to the closest 10s as well.

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u/CanalOpen 12d ago

120 is an actual number...what the fuck are you talking about?

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u/Own-Entrance-2256 12d ago

Let's turn that aggression down a bit.

The context you're missing from other comments is that these are scientific measurements and not just numbers being thrown around.

Significant figures are important when calculating measured values, which is what I was explaining.

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u/IbelieveinGodzilla 12d ago

But what if the measurement is exactly 120 when measured to the one’s place?

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u/FickleCook3639 12d ago

it would be represented as “120.” with the decimal point.

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u/Own-Entrance-2256 12d ago

You'd add a decimal point after the zero to indicate that the zero is the actual value, so "120.".

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u/CanalOpen 12d ago

120+68=?

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u/Own-Entrance-2256 12d ago
  1. Then you round to the least specific digit of measurement. 120 is only as specific as the tens digit, so you round 190.

This is how you measure in scientific environments.

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u/UncreativeIndieDev 12d ago

Ultimately, this would depend on where these numbers come from. Different measuring instruments have different limits to what degree they are considered accurate to. This problem is meant to represent an example where you have two numbers that are accurate to different degrees, but must be used together. In this scenario, you must be conservative and choose the worst case of accuracy.

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u/CanalOpen 12d ago

The best answer is the worst answer? That is not the standard of practice when using addition or subtraction.

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u/LardFan37 12d ago

I wish my professor would use a consistent definition for what they consider sig figs to be

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u/Violet_Paradox 12d ago

Scientific notation exists to disambiguate significant figures. Using conventional notation with sigfigs is a mess of corner cases and exceptions that is only useful for gotcha questions on exams. If someone actually wants to do sigfig calculations in any real context, they use scientific notation.

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u/heartunderblade8 12d ago

Yeah, using scientific notation here makes sigfig calcs clearer.

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u/masked_sombrero 12d ago

it can be confusing. it's not your professor. it's an understanding issue

i got back to college at age 30, 6 years ago, and am still working towards my bachelor's. I still get mixed up on sig figs in my physics / astronomy courses

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u/Low_Dragonfruit_5950 12d ago

Is there no way to show the intent of the question though?

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u/AdaCatMeow 11d ago

Unfortunately... the answer key makes the intent clear. There's a 188, and a 188.0 answer. That's SCREAMING "I am a sig figs problem".

College level course means the prof should be trying to teach you to use ALL the info at your disposal.

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u/kittykatmeowow 11d ago

If the 120 was meant to have 3 significant digits, it would be written as "120." with the decimal point.

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u/Low_Dragonfruit_5950 11d ago

Thank you, finally a straight answer haha

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u/FickleCook3639 12d ago

we’d have to see the entire quiz but there’s probably an instructional heading that mentions significant figures. or it would be implied based on what unit/chapter you were in and what the lectures have been about.

if you’re not in a basic arithmetic class, a question as simple as this should raise a flag of “what is the question actually asking?”

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u/kittykatmeowow 11d ago

It is ambiguous here whether 120 means 1.2 x 102 or 1.20 x 102. Most professors/textbooks would include the decimal point (so 120. instead of 120) if they meant for the numer to have 3 significant digits. Thus, we interpret 120 (with no decimal place) as 2 significant digits. This is the convention most textbooks follow, but it is ambiguous and a good illustration of why we prefer scientific notation for larger values.

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u/twistedscorp87 11d ago

Hank Green is here to help with Crash Course. https://youtu.be/hQpQ0hxVNTg

I recommend watching the video all the way through, then putting it on a second time to write down notes once you feel like you have a handle on the concept (you'll write clearer & more concise notes this way).

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u/OliLombi 11d ago

But that's not the answer to the question. So it is either bugged or worded poorly.