Re: conditional probability

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Originally Posted by

**kkoutsothodoros** I am working on this problem: There are two precalculus sections at West High School. Mr. Abel's class has 12 girls and 8 boys, while Mr. Bonitz's class has 10 girls and 15 boys. If a West High precalculus student chosen at random happens to be a girl, what is the probability she is from Mr. Abel's class? [the answer is not 12/22].

My reasoning is as follows: P(A|G) = P(G|A)*P(A)/P(G) = (12/20)*(20/45)/(22/45) which is, unfortunately, giving me 12/22. i don't see any flaws in my reasoning.

There is no flaw in your reasoning.

The answer is $\displaystyle \frac{12}{22}$. **Why do you think that it is not?**

Re: conditional probability

Hi thanks! the book specifically gives the hint: "the answer is not 12/22!" but it's what i get!! i've done this before. i guess i should be a little more confident in my logic.

Re: conditional probability

Quote:

Originally Posted by

**kkoutsothodoros** Hi thanks! the book specifically gives the hint: "the answer is not 12/22!" but it's what i get!! i've done this before. i guess i should be a little more confident in my logic.

**Are you sure that you have copied the question exactly**?

If it were given that student is from Able's class, what is the probability it is a girl? That would be different.

Re: conditional probability

Thanks again. The answer in the back of the book is 3/5 which agrees with your last comment. i'm a math tutor and i used to be an actuary so conditional probability is something I've thought about often. when i got 12/22 and the problem specifically stated it WASN'T it was a bit embarrassing!