Consider a sequential service system consisting of two servers, A and B$.$ Arriving customers will enter this system only if server A is free. If a customer does enter, then he is immediately served by server A$.$ When his service by A is completed, he then goes to B if B is free, or if B is busy, he leaves the system. Upon completion of service at server B$,$ the customer departs. Assume that the Poisson arrival rate is two customers an hour and that A and B serve at respective exponential rates of four and two customers an hour