A self-service store employs one cashier at its counter. Fifteen customers arrive on an average

every 5 minutes. While the cashier can serve nine customers in 5 minutes. Assuming Poisson

distribution for the arrival rate and exponential distribution for service time. Find

(a) Average number of customers in the system

(b) Average number of customers in the queue or average queue length

(c) Average time, a customer spends in the system

(d) Average time, a customer waits before being served