18.15 Birth order. A couple plan to have three children. There are eight possible arrangements of girls and boys. For example, GGB means the first two children are girls and the third child is a boy. All eight arrangements are (approximately) equally likely.
(a) Write down all eight arrangements of the sexes of three children. What is the probability of any one of these arrangements?
(b) What is the probability that the couple’s children are two girls and one boy?