A super market has two girls ringing up sales at the counters. If the service time for each customer is exponential with mean 4 minutes and if people arrive in a Poisson fashion at the counter at the rate of 10 per hour, then calculate

a) The probability of having to wait for service.

b) the expected percentage of idle time for each girl.

c) If a customer has to wait find the expected length of his waiting time.