need help solving problem below:

Customers arrive at a service station with the intensity A = 5 per hour. Assume that customer arrivals form a Poisson process, X = {X (t) : t 2 0}. Introduce for any natural 11:, such that k 2 1, the arrival times, Wk =min[t 20|X(t) =k]. 1. Given that the eighth customer arrived exactly at the end of third hour, or W8 = 3, nd the conditional density of the forth arrival time (W4). 2. Evaluate E [W4|W8 = 3] and Var [W4|W8 = 3]