Airport Massage Station a Small Company Just Opened a New Massage Station at Massage

Multiple Choice

Airport Massage Station
A small company just opened a new massage station at Philadelphia airport. The company has a stand that offers massages to travelers. Customers can select a length of massage between 5 and 20 minutes and there is a unique rate of $30 independently of the length selected by customers. The average length of massage requested by customers is of 15 minutes with standard deviation of 10 minutes.
There are four employees delivering massages. The average number of potential customers requesting a massage is of 20 per hour. The inter-arrival times are assumed to be exponentially distributed.
If no spot is available when the customer arrives, s/he leaves in order not to risk missing her/his flight.
-What is average number of customers being serviced per hour at the massage station? Circle the closest answer.

A) 7
B) 8
C) 9
D) 10
E) 11
F) 12
G) 13