Take the tour ×  You know that a couple has two children. You go
to the couple's house and one of their children, a young boy,
opens the door. What is the probability that the couple's other
child is a girl?  If you list all possibilities for the sexes of
two children, BB, BG, GB, GG, you see that 2 of the 3 pairs that
have B (for boy) in them also have a girl, so the answer one
could argue is 2/3.  On the other hand, one could argue that the
answer is 1/2, since the probability that any one child is a
girl is 1/2, and intuitively (?) should be independent of the
gender of its siblings.  Some background to possibly justify
posting it here: the question was asked at an interview for an
actuarial/insurance type position, and the interviewer was the
answer was 2/3, whereas my friend who was being interviewed (and
has a masters in math) thought the answer was 1/2, even after
the interviewer explained his logic. My friend felt that the
interviewer wasn't taking into account the fact that it is not
equally likely t