need help with problem below :

Problem 5: 15 points (= 5 + 10) Assume that arrivals form a Poisson process N={N(t): 1320} with sojourn times being exponentially distributed with mean equal to 40 minutes. Let Wk denote the kth arrival time. 1. Derive the distribution of a difference, W5 W2 2. Determine expected value of _ W5 W2 V W2