Class A contains 5 girls + 10 boys= 15 students. Class B contains 12 girls + 13 boys= 25 students. Teachers randomly pick a student from Class B and send them over to Class A; they then randomly pick a student from Class A. We're trying to find the probability that this student is a boy.

To answer this question, I calculated several conditional probabilities: $P(M|A)=10/15$
$P(F|A)=5/15$ $P(M|B)=13/25$ $P(F|B)=12/25$

What I tried next is the following, but I'm not sure this is the right way to go: P(M|A new)=(10+(13/25))/16=0.6575. Can you add the probability almost as a new student? If not, is there a simpler/more correct way to answer this question?


Your answer is correct.

The usual way to solve it is $\left(\frac{13}{25}\frac{11}{16} + \frac{12}{25}\frac{10}{16}\right)$

In the way you have solved, $(10+ \frac{13}{25})$ is the expected number of boys in class A after the switch.


$\frac{13}{25}$ you have added is not a probability, view it as $\frac{13}{25}\times 1 + \frac{12}{25}\times 0$, the expected number of boys added to class A.

  • $\begingroup$ You are welcome ! $\endgroup$ – true blue anil Feb 17 '16 at 4:43

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