3 Four doctors work in a hospital emergency room that handles three types of patients. The time a doctor spends with each type of patient is exponentially distributed, with a mean of 15 minutes. Interarrival times for each customer type are exponential, with the average number of arrivals per hour for each patient type being as follows: type 1, 3 patients; type 2, 5 patients; type 3, 3 patients. Assume that type 1 patients have the highest priority, and type 3 patients have the lowest priority (no preemption is allowed). What is the average length of time that each type of patient must wait before seeing a doctor?