1. The problem statement, all variables and given/known data A king comes from a family of two children. What is the prob that the other child is his sister? 3. The attempt at a solution I have the correct answer below, I just want to check my argument is good. I think the question is badly worded because of the use of the word 'sister'. In order to get the correct answer, I believe you have to assume this means 'girl', otherwise you would have to consider whether or not the other child is the king's brother, sister, cousin, step-cousin etc.. Anyhow, what I said was P(other child girl|one child is a boy) = P(other child girl and one child boy)/P(one child is a boy). Writing down the possible ways: BB, BG, GG gives the numerator as 1/3 and the denominator is 1/2 since we have either BG or BB. (1/3)(1/2) = 2/3. Is this ok? It gives the correct answer but I am not sure if I can assume sister just means girl.