How much time does a customer spend at service desk B either waiting for help or being helped by the clerk?

Sharp Discounts Wholesale Club has two service desks (call them A and B) with one server at each desk. There is one desk at each entrance of the store. Customers arrive at each service desk at an average of one every six minutes following a Poisson distribution. The average service time at each service desk is 4 minutes per customer following the exponential service distribution