Customers arrive at a grocery store checkout at a rate of six per 30 minutes during the hours of 5 p. m. and 7 p. m. during the workweek. Let C be the number of customers arriving at the checkout during any 30-minute period of time. The management of the store wants to determine the frequency of the following events. Compute the probabilities of these events:
a. No customers arrive.
b. More than six customers arrive.
c. At most three customers arrive.