Which one of the following is the average number of customers in the line, including the customer being served?

Question

If average arrival rate in a queue is 6/hr, and the average service rate is 10/hr, which one of the following is the average number of customers in the line, including the customer being served?                    

(a) 0.3                             (b) 0.6

(c) 1.2                             (d) 1.5