The probability that they are both girls is 1/2. We know that one child is a girl. This does not affect the probability of what the other child is. If you are equally likely to have a boy or a girl, having another child does not make any change to that. In fact, there are only 3 possible combinations of gender, two boys, two girls, and one of each. Since two boys are not possible, there is a 1/2 probability of the other child being a girl.