12. A group of m customers frequents a single-server station in the following manner. When a customer arrives, he or she either enters service if the server is free or joins the queue otherwise. Upon completing service the customer departs the system, but then returns after an exponential time with rate θ. All service times are exponentially distributed with rate μ.

(a) Find the average rate at which customers enter the station.

(b) Find the average time that a customer spends in the station per visit.