1. The problem statement, all variables and given/known data Suppose you choose a random family with two children. One of them is a girl. What is the probability that both children are girls? Derive your answer by explicitly constructing an outcome space and a probability measure, and naming the relevant events in terms of this outcome space. You may assume that the probability that a child is male or female is 1/2, independently of the gender of another child. 3. The attempt at a solution Alright, so my question is fairly straightforward; what situation am I dealing with here? If I can trust wikipedia, there are two common ways of phrasing the issue, namely From all families with two children, at least one of whom is a boy, a family is chosen at random. This would yield the answer of 1/3. From all families with two children, one child is selected at random, and the sex of that child is specified. This would yield an answer of 1/2. Now, I understand the reasoning behind both of them, but I can't decide in which category my version of the question falls. It seems to have characteristics of both of them. A random family of two children is chosen, so that sounds like the 1/2 case. But a child is not identified or anything, so that makes it sound more like 1. Can anyone help me out?