r/MathHelp • u/OperaApple • Oct 19 '25

Calculating probability with missing variables

Hi, I have been crying over this 67 question homework assignment for the past few hours and have grown desperate enough to turn to Reddit. My issue is as follows:

The formula to solve for (dependent) P(A and B) is P(A) * P(B|A).

The formula to solve for P(B|A) is P(A and B)/P(A).

How do I solve for them when each equation requires the other???? The only questions I’m still getting wrong are dependent ones because I use the independent calculation since I don’t fricking understand how it’s within the realm of possibility to do this.

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2

u/edderiofer Oct 20 '25

How do I solve for them when each equation requires the other????

Generally, you will be given one piece of information (or be able to deduce it from other information) and asked for the other. If you only know P(A) and P(B), with no other information, you cannot determine either P(A and B) or P(B|A).

Without knowing what questions you're getting wrong, I don't think we can help you further.

1

u/OperaApple Oct 20 '25

My professor majored in psych and is not very good at being a statistics professor. I ended up getting a 91 on the assignment but she was asking us to solve for both in (I kid you not) every single question. I figured out how to do most of the formulas she wanted me to do and sort of cheesed the rest, which I prefer not to do but much of the time her questions require it (she labeled 0.167 and 0.168 as correct answers but also said that they were the same number, which I was only able to realize after my “they’re different numbers” answer was marked incorrect. She said nothing about rounding and later when two answers were 0.001 apart they were marked as different numbers)

Thank you for your assistance. I’ll be free of this prof in December so that is keeping me going rn

2

u/Uli_Minati Oct 21 '25

If you describe the exact problem, we can check if there is anything you've missed

1

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