During the early morning hours, customers arrive at a branch post office at an average rate of 57 per hour (Poisson), while clerks can provide services at a rate of 19 per hour. If clerk cost is $45.58 per hour and customer waiting time represents a cost of $43 per hour, how many clerks can be justified on a cost basis . 4 b. 6 . 7 d. 8 e. 5