18. In Exercise 17, let Xi denote the number of times that the ith type outcome occurs, i = 1,...,r.

(a) For 0 ≤ j ≤ n, use the definition of conditional probability to find P(Xi =

xi,i = 1,...,r − 1|Xr = j).

(b) What can you conclude about the conditional distribution of X1,..., Xr−1 given that Xr = j?

(c) Give an intuitive explanation for your answer to part (b).