Your neighbours has two children, one of which is a girl. What is the probability of the other child to be a girl? This is oft-mentioned but with this wordin

Your neighbours has two children, one of which is a girl. What is the probability of the other child to be a girl? This is oft-mentioned but with this wording it's ambiguous. Here are three possible interpretations: 1. Your neighbor has two children. We observed both and found that there was exactly one girl. What is the probability that both are girls? (Answer: 0) 2. Your neighbor has two children. We observed one and found that she was a girl. What is the probability that the other (unobserved) child is a girl? (Answer: 1/2) 3. Your neighbor has two children. We observed both and found that there was at least one girl. What is the probability that both are girls? (Answer: 1/3) Interpretation #1 is kind of silly but it serves to illustrate the ambiguity of the plain English wording. Interpretation #2 is how most people interpret the problem, and it's bolstered by the use of the word "other" in your prompt. For there to be an "other" child, there must have been a specific girl observed originally, rather than a general observation that there was at least one girl. Interpretation #3 is how you intended the problem.