Jobs arrive at a system according to a Poisson process with rate 10 per hour. The service requirement of each job has an exponential distribution with mean 5 minutes. For use of the system, each arriving job is charged one dollar, with a 20 cent rebate if the job has to wait before entering service (the jobs wait in one queue). A server costs 1 dollar per hour to operate (this is a fixed cost, and does not depend on the server utilization). To maximize expected profit for the system, how many servers should be used?