Consider a small self-service car wash that has cars arriving with exponentially distributed inter arrival times at a mean rate of 15 per hour. The car wash has 3 washing bays, each equipped with a pressure spraying hose that the customer uses to wash off the car. Washing time, in either bay, is exponentially distributed with a mean washing time of 4 minutes. The car wash has very little space and can accomodate only 2 washing cars in addition to those being washed. If all 2 spots are occupied arriving cars leave and go elsewhere to be washed.
1. What is the expected number of cars at the car wash?