7.21 Consider a two-priority queueing system in which priority class 1 (i.e., high-priority) customers arrive according to a Poisson process with rate two customers per hour and priority class 2 (i.e., low-priority) customers arrive according to a Poisson process with rate five customers per hour. Assume that the time to serve class 1 customers is exponentially distributed with a mean of 9 min, and the time to serve class 2 customers is constant at 6 min. Determine the following:

a. The mean waiting times of class 1 and class 2 customers when nonpreemptive priority is used.

b. The mean waiting times of class 1 and class 2 customers when preemptive resume priority is used.

c. The mean waiting times of class 1 and class 2 customers when preemptive repeat priority is used.