90. The number of accidents in each period is a Poisson random variable with mean 5. With Xn, n  1, equal to the number of accidents in period n, find E[N] when

(a) N = min (n: Xn−2 = 2, Xn−1 = 1, Xn = 0);

(b) N = min (n: Xn−3 = 2, Xn−2 = 1, Xn−1 = 0, Xn = 2).