ABC Oil Change, being the only one in this small town, has a thriving business. There are customers arriving every 6 minutes. The facility can change oil with a mean service time of 15 minutes and a standard deviation of 15 minutes. There are three oil change bays with one worker in each bay. See figure below for its operation. Assume the coefficient of variation equals to one for the inter-arrival time. What is the average time a car needs to wait to get an oil change?