15. A supermarket has 2 exponential checkout counters, each operating at rate 6 per unit time. Customer inter-arrival times are exponential with rate 8 per unit time. One FIFO (first-in-first-out) queue feeds both counters. Counter A is operated by a permanent checker and counter B is operated by a part-time checker who opens counter only if there are 3 or more customers in the system. Customers are promptly directed to another section of the store (not of interest in this problem instance) for service whenever there are up to 6 cus tomers on this queue. For the purpose of doing personnel scheduling, (a) model this supermarket situation as a CTMC in rate form, and (b) thereafter convert your CTMC into one in sojourn-time form