a$.$ The utilization rate for this service system is calculated by dividing the average arrival rate by the average service rate. In this case, it would be

or $75\%.$

b$.$ The average time before the facility can return a breakdown to service is the reciprocal of the service rate, which is

days.

c$.$ The average time a vehicle spends waiting for service is calculated by multiplying the utilization rate by the average time in the system. The average time in the system is

days. Therefore, the average waiting time is

days.

d$.$ The average number of vehicles in the system at any one time is the arrival rate times the average time in the system, which is

vehicles.