4 An average of 120 students arrive each hour (interarrival times are exponential) at State College’s Registrar’s Office to change their course registrations. To complete this process, a person must pass through three stations. Each station consists of a single server. Service times at each station are exponential, with the following mean times:

station 1, 20 seconds; station 2, 15 seconds; station 3, 12 seconds. On the average, how many students will be present in the registrar’s office for changing courses?