Why does the self-correcting mechanism stop working when the policy rate hits the zero lower bound?

Customers arrive at rich dunn's styling shop at a rate of 3 per hour, distributed in a poisson fashion. rich can perform an average of 5 haircuts per hour, according to a negative exponential distribution.a) the average number of customers waiting for haircuts= customersb) the average number of customers in the shop= customersc) the average time a customer waits until it is his or her turn= minutesd) the average time a customer spends in the shop= minutese) the percentage of time that rich is busy= percent