How much time is a customer expected to spend waiting in line and being checked out?

The local grocery store consists of two cashiers. The customers arrive at the checkout according to the Poisson distribution and the service times are based on negative exponential distribution. The average customer service time is 4 minutes and the average time between the arrivals of successive customers is 3 minutes. Assume that customers equally divide themselves between the two cashiers.

Answer: Wq = .13 hour or 8 minutes and W = 0.2 hour or 12 minutes