Solution to the Two Children Puzzle

There are two equally probable possibilities:

1) The younger child is a boy.
2) The younger child is a girl, and therefore both children are girls.

It can therefore be seen that, given the information we have, the probability of Mr Kidd having two girls is 1/2.

Now consider this...

Professor Ravenscroft also has two children. At least one of them is a boy.

What is the probability that both his children are boys?

SOLUTION