GazzaTheBook and noobsauce
We know there are 2 kids and one of them is a girl. So all possible cases 3; G-G, G-B, B-G. These 3 cases are equally likely, right?
Number of events favourable to the outcome is 1, G-G, right? So the probability is 1/3.
Apologies for the "utter rubbish" comment.
I don't disagree wtih your rationale above. It is perfectly plausible and technically correct given your question. But that is not the question in the problem.
The question is: They have two kids, one of them is a girl, what is the probability that the other kid is also a girl?".
We are asked to evaluate whether "the other kid" is a girl. In this instance I believe it is incorrect to apply your reasoning because you are analysing the probability outcomes of 2 births. The question doesn't ask us to do that, it merely asks us to work out the probability of a nominated child (the other kid) being a girl.
The "best" answer we can give is 1/2 as I have demonstrated in my previous post.