A self-service car wash has four sections. In each one, the customers can wash and wax theircars. Customers arrive at the service following a Poisson process with a mean rate of 15cars/hour. If there is no room for waiting, cars must leave the service. Apparently, the time required to service a car is distributed exponentially with a mean of 12 minutes. Determine: a) Average number of cars in the service at any moment