In a certain village, it is traditional for the eldest son (or the older son
in a two-son family) and his wife to be responsible for taking care of his
parents as they age. In recent years, however, the women of this village, not
wanting that responsibility, have not looked favorably upon marrying an eldest
son.
(a) If every family in the village has two children, what proportion of all
sons are older sons?
(b) If every family in the village has three children, what proportion of all
sons are eldest sons? Assume that each child is, independently, equally likely
to be either a boy or a girl.