This is easier to understand if you start by listing all of the possibilities. Two boys: BB Two girls: GG Boy, then girl: BG Girl, then b

This is easier to understand if you start by listing all of the possibilities. Two boys: BB Two girls: GG Boy, then girl: BG Girl, then boy: GB So there are four possibilities: (BB, GG, BG, GB). If you know that one of the children is a girl, then BB is impossible and you can remove it from the list. This leaves only 3 possibilities (GG, BG, GB). This is the set you use to calculate the probability and there are two ways out of three that the person could have both a boy and a girl: BG and GB That gives you 2/3 or 67%