I've always thought of it this way.. imagine that there is one car wash in the city and all the cars have to go through it. The car wash can only service cars at a certain rate, say 5 cars an hour? Unfortunately the cars can only wait at the car wash for so long before they have to move on (that is they can't sit there forever and wait), so cars are moving through the car wash at a certain rate independent of how fast the car wash can wash cars. As cars come in they get washed, or if they've waited for too long they move on and don't get washed. If cars come in very quickly, obviously fewer will get washed and will just end up passing on through unwashed and so the dirty car ends right back on the streets. If the cars come in slower at a rate that the car wash can handle then more cars will get washed. If it is a perfect day where the rate of cars coming in equals the rate of the cars being washed then all cars will get washed. Also, keep in mind that this is the only car wash in the city so all cars must eventually get washed, so eventually those dirty cars that had to pass through unwashed last time will eventually come back and try again to get washed. The faster they can turn around and get off the streets and come back to the car wash the better their chances of getting washed within a small time frame, but that also means they can't spend as much time at the car wash.
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