sachi and lachi are a married couple (dont ask me who he is and who she is)! They have two kids, one of them is a girl. Assume safely that the probability of each gender is 1/2. What is the probability that the other kid is also a girl?

This is a classic conditional probability puzzle. For a two-child family, the equally likely possibilities are: BB, BG, GB, GG (where B=boy, G=girl, and order matters). Given that at least one child is a girl, we eliminate BB, leaving three equally likely cases: BG, GB, GG. Only in GG are both children girls. Therefore, P(both girls | at least one girl) = 1/3. Option A is correct. The common mistake is thinking the answer is 1/2, which ignores that BG and GB are distinct cases.