Suppose that there exist N families on the earth and that the maximum number of children a family has is c. For j = 0, 1, 2, ... , c, let a1 be the fraction of families with j children (Ec•:, a1 = 1). A child is selected at random from the set of all children in the world. Let this child be the Kth born of his or her family; then K is a random variable. Find E(K).