AVI supplies vended food to a large university. Because students kick the machines at every opportunity, management has a constant repair problem. The machines break down on an average of three per hour, and the breakdowns are distributed in a Poisson manner. Downtime costs the company $25/hour per machine, and each maintenance worker gets $4/hour. One worker can service machines at an average rate of five per hour, distributed exponentially; two workers working together (as one team) can service seven per hour, distributed exponentially; and, a team of three workers can do eight per hour, distributed exponentially. What is the optimum maintenance crew size?