試題詳解

22. (5%) Consider the set of all families with two children. Randomly select one family. Assume that in a two-child family all sex distributions are equally likely. In addition, assume that there are n distinct names for girls. Assume that in families with one girl, each name is equally likely to be assigned to the girl. Assume that in families with two girls, each pair of two distinct names is equally likely to be assigned to the two girls. It is known that the randomly selected family has one child called Mary.What is the probability that the other child of the family is a girl?
(A) 2/3
(B) 1/2
(C) 1/3 
(D) 1/4
(E) 1/5