EXAMPLE 5.28

Blood types of children. Each child born to a particular set of parents has probability 0.25 of having blood type O. If these parents have five children, what is the probability that exactly two of them have type O blood?

The count of children with type O blood is a binomial random variable X with n = 5 tries and probability p = 0.25 of a success on each try. We want P(X = 2).