The following information pertains to telephone calls to a call centre on a typical workday. Assume Poisson arrivals and Exponential service durations.

a. Determine the average time callers wait to have their calls answered for each period and the probability that a caller will have to wait during each period.
b. The call centre has seven leased phone lines (trunks). What is probability that a phone call will receive a busy signal during each of the three periods?