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.

What is the average number of customers who are on hold?