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).