A local business person is considering building a self-service car wash with some number of stalls.

It is estimated that cars will arrive to the car wash at the rate of two per hour. The average time taken to wash a car is estimated to be 20 minutes. Assume that interarrival times and service times are exponentially distributed.

Customers in this particular town do not like to wait. If an arriving customer finds that all stalls are in use, then she will drive away and her business is lost. That is to say, if all stalls are busy then a customer does not wait and leaves with probability one.

Determine the minimum number of stalls required so that the long-term proportion of lost customers is less than p = 0.15