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A small barbershop is operated by a single barber. it has room for at most two customers. potential customers arrive as a poisson process with a rate of three per hour, and the successive service times are independent exponential random variables with mean 0.25 hour. find: (a) the average number of customers in the shop. (b) the proportion of potential customers that enter the shop (in the long run). (c) if the barber could work twice as fast, how much more business could he do?