One of the features of a new machine shop will be a well-stocked tool crib. The manager of the shop must decide on the number of attendants needed to staff the crib. Attendants will receive $20. Mechanics’ time will be worth $30 per hour. Based on previous experience, the manager estimates requests for parts will average 18 per hour with a service capacity of 20 requests per hour per attendant. How many attendants should be on duty if the manager is willing to assume that arrival and service rates will be Poisson-distributed?