In a single phase single server service process where both interarrival time and service times are exponentially distributed, it can be assumed that the total time that a customer spends in the process is also exponentially distributed. Suppose the total time the customers spend in a pharmacy is exponentially distributed with mean of 4 minutes. The pharmacy has promised to fill all prescriptions in 9 minutes.

Question: 82% of customers are served in less than what time interval?

The potential answers are:

A: 6.86 minutes.

B: 9.83 minutes.

C: 12.72 minutes.

D: 4.83 minutes.

E: 7.33 minutes.