4.4. Suppose that incoming calls to an office follow a Poisson process of rate A = 6 per hour. If the line is in use at the time of an incoming call, the secretary has a HOLD button that will enable a single additional caller to wait. Suppose that the lengths of conversations are exponentially distributed with a mean length of 5 minutes, that incoming calls while a caller is on hold are lost, and that outgoing calls can be ignored. Apply the results of Section 4.1 to determine the fraction of calls that are lost.