exponetial distributions

4. In a police department, only selected clerks are allowed to locate files in the vault. When police officer want to access a file, they must queue until a clerk become available. Suppose interarrival times between the police officers requests for files are iid exponential with mean 5 min, the time for a clerk to locate a file is also an exponential RV with the mean 7.5 min. A clerk's salary is $10 per hour, a police officer's salary is $15 per hour. The police department can hire up up to three clerks. How many should they hire to minimize delay and service costs