Just so no one loses any sleep over this.
A family has two children, there are 4 possible scenarios:
Boy Boy
Boy Girl
Girl Boy
Girl Girl
Since one of the children is a girl there are only three possible scenarios:
Boy Girl
Girl Boy
Girl Girl
Of those three scenarios only one has the desired outcome.
Therefore the probability is 1/3.
QED
(I remember this problem from a probability course. There is actually a massive machlokes between some really great math nerds, I’m not saying what side I’m on, but I knew that if truthsharer was asking this question he was probably not looking for the obvious answer.)
See you all in the morning.