During the Early Morning Hours, Customers Arrive at a Branch

Multiple Choice

During the early morning hours, customers arrive at a branch post office at the average rate of 45 per hour (exponential interarrival times) , while clerks can handle transactions on an average of 4 minutes each (exponential) .
What is the minimum number of clerks needed to keep the average time in the system under 5 minutes?

A) 2
B) 3
C) 4
D) 5
E) 6