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 leaves. Assuming that the arrival rate is two customers an hour, and that A and B serve at respective rates of four and two customers an hour. $($a$)$ what proportion of customers enter the system? $($b$)$ what proportion of entering customers receive service from B$?($c$)$ what is the average number of customers in the system? $($d$)$ what is the average amount of time that an entering customer spends in the system? Can you write the balance equation and the diagram