28. Customers arrive at a barber's shop according to Poisson distribution. The average time between successive arrivals is 6 minutes. There are three barbers, all of same efficiency. The service time of the customers is exponentially distributed with a mean equal to 10 minutes per customer. Find

(a) the expected number of customers in the shop,

(b) the expected time a customer spends in the shop, (

c) the average time a customer has to wait in the queue, and

(d) the expected number of barbers idle.