Question

A couple has two children, one of the child is a boy. What is the probability that the other child is also a boy?

Hint: Its not 1/2.

Answer 1

Possible outcomes: B - Boy & G - Girl
BB
BG
GB
GG
GG is out of the question because we know that one of the child is a boy so the required probability of BB happening is = 1/3.

Answer 2

1/3 is the answer
The following are possible combinations of two children that form a sample space in any earthly family:

Boy - Girl
Girl - Boy
Boy - Boy
Girl - Girl

Since we know one of the children is a boy, we will drop the girl-girl possibility from the sample space.
This leaves only three possibilities, one of which is two boys. Hence the probability is 1/3