The rst time they get one of the two sexes. After that they are waiting for the event of probability 1/2 and expected value of 2, that gives them the other one, so the mean is 1 + 2 = 3. Solution The rst time they get one of the two sexes. After that they are waiting for the event of probability 1/2 and expected value of 2, that gives them the other one, so the mean is 1 + 2 = 3..