Please show all work / steps. Thank you.

Customer calls are handled by one employee at a small business' call center. The time to answer each call is exponentially distributed, with a mean of two minutes per call. Requests for calls come in at a mean rate of 0.4 per minute, which can be assumed to be Poisson distributed. If the employee is helping a customer, others who call will wait on hold until the employee becomes free. All calls are served in first come first served order. Determine the following: a. What is the average number of customers who are on hold