standard deviation

Cars arrive for servicing at Al’s Repair Shop according to a Poisson distribution at a mean rate of 4 per day. The average time A1 takes to work on a car is 2 hours, and the standard deviation is 15 minutes. A1 is the only employee, and he works 10 hours a day. Determine the average number of cars that will be waiting for A1 to begin working on them?

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