Suppose you are managing a service system in which customers arrive at a rate of two per minute according to a Poisson process. You are investing in a single machine that provides the service and does so with exponential service time. Your objective is that the average time a customer spends in the system be less than or equal to five minutes. How fast of a machine do you need?

I googled "exponential service time" the first choice (5.2 exponential service) may have the equations you need in appendix A. as I am clueless in this area I hope I have been helpfull :)