Hmm.
1) I'd say it's 1/4. It's 1/2 that the first child is a boy, and then another 1/2 that the second child is also a boy.
2)We discard the possibility that the first and second child are both girls, so we are left with three options, thus 1/3.
3) Erm... I've got to think a bit more about this
