The expected length of the queue will be

Question

In a single serve infinite population queuing model, arrivals follow a Poisson distribution with mean $\lambda$ = 4 per hour. The service times are exponential with mean service time equal to 12 minutes. The expected length of the queue will be                                        

      (a) 4                                                                    (b) 3.2

      (c) 1.25                                                               (d) 24.3