The Madrid Mist outlet store at Chiswell Mills sells discount luggage and does

most of its daily business in the evening between the hours of 6 and 9 pm. During this time, customers arrive at the checkout desk at a rate of one every 2 minutes following a Poisson distribution. The checkout operation takes an average of 3 min- utes per customer and can be approximated well by an exponential distribution. Madrid Mist’s corporate policy is that customers should not have to wait longer than 1 minute to begin the checkout operation.

a. What is the average service rate per minute? b. What is the average arrival rate per minute? c. What would happen if the store operated a single checkout station during the

time period in question? d. How many checkout stations should the store plan to operate during this time

period to stay within the corporate policy on checkout operations?

The main one I need Answering is D 

thank you