A small company just opened a new massage station for travelers at Denver International Airport . Customers can select a length of massage between 5 and 20 minutes , but the rate is fixed at $ 30 independent of length . The average length of massage is 15 minutes with a standard deviation of 10 minutes . There are four employees available to give massages . The average number of customers requesting a massage is 20 per hour . The interarrival times are assumed to be exponentially distributed . If no spot is available when the customer arrives , the traveler leaves in order not to risk missing their flight . a. What is the average number of customers serviced per hour.