Ok, here's the problem.
The probability of a child inheriting a certain disease is 1/2. In a two child family, what is the probability that the younger child will inherit this disease given that the older child has already inherited it?
Here's what I've come up with, but I don't feel that it's right.
A=Older child is affected B=Younger child is affected
P(A) = 1/2 P(B) = 1/2
P(B│A)= (P(B∩A))/(P(A))= (1/4)/(1/2)=2/4=1/2
I guess this makes sense because the probability of each child getting the disease is independent of the other one having the disease but a part of me feels that the answers should be 1/4 because according to the problem, the older child already has the disease meaning that P(A) = 1