It's a Beautiful Sunny Day After a Week of Rainy

Essay

It's a beautiful sunny day after a week of rainy weather and both of the local car washes are busy. One car wash is fully automated, touchless, and can wash a vehicle of any size to a dazzling luster in 5 minutes flat. The other car wash is a fund raiser staffed by eager amateurs that work on one car at a time and can finish it in about 4 minutes (their time is governed by the Poisson distribution). Customer arrivals at both car washes are Poisson distributed with a mean of 10 per hour. How long does it take the average customer to work his way through either system? How many customers are in the systems on average?