Old thread, and masses of discussion, but to simplify - There are 4 equally likely combinations of 2 children: Boy/Boy; Girl/Girl; Boy/Girl; Girl/Boy. The odds of the second child being a girl depend upon how the OP is interpreted and the possible combinations (green) each interpretation allows.
1. They have two kids, [only] one of them is a girl.
Boy/Boy Girl/Girl Boy/Girl Girl/Boy - Trick question. In the 2 possible combinations the other child must be a boy. Probability 0.
2. They have two kids, [the first] one of them is a girl.
Boy/Boy Girl/Girl Boy/Girl Girl/Boy - There are 2 possible combinations where the first child is a girl. In 1 the other child is too. Probability 1/2.
3. They have two kids, [at least] one of them is a girl.
Boy/Boy Girl/Girl Boy/Girl Girl/Boy - Without the Boy/Boy combination 3 remain, in 1 of which the other child is a girl. Probability 1/3.
