A small PBX serving a startup company can only support five lines for communication with the outside world. Thus, any employee who wants to place an outside call when all five lines are busy is blocked and will have to hang up. A blocked call is considered to be lost because the employee will not make that call again. Calls to the outside world arrive at the PBX according to a Poisson process with an average rate of six calls per hour, and the duration of each call is exponentially distributed with a mean of 4 minutes.

a. What is the probability that an arriving call is blocked?

b. What is the actual arrival rate of calls to the PBX?