13.8. Green Valley Airport has been in operation for several years and is beginning to experience flight congestion. A study of airport operations revealed that planes arrive at an average rate of 12 per hour, with a Poisson distribution. On the single runway, a plane can land and be cleared every 4 minutes on average, and service times have a negative exponential distribution. Planes are processed on an FCFS basis, with takeoffs occurring between landings. Planes waiting to land are asked to circle the airport.