City Cab, Inc., uses two dispatchers to handle requests for service and to dispatch the cabs. The telephone calls that are made to City Cab use a common telephone number. When both dispatchers are busy, the caller hears a busy signal; no waiting is allowed. Callers who receive a busy signal can call back later or call another cab service. Assume that the arrival of calls follows a Poisson probability distribution, with a mean of 40 calls per hour, and that each dispatcher can handle a mean of 30 calls per hour.

a. What percentage of time are both dispatchers idle?

b. What percentage of time are both dispatchers busy?

c. What is the probability that callers will receive a busy signal if two, three, or four dispatchers are used?

d. If management wants no more than 12% of the callers to receive a busy signal, how many dispatchers should be used?