In Problem 13.7 on page 531, you used the total number of customers in the store to predict the waiting time at the checkout counter. The data are stored in Supermarket.
a. Construct a 95% confidence interval estimate of the mean waiting time for all customers when there are 20 customers in the store.
b. Construct a 95% prediction interval of the waiting time for an individual customer when there are 20 customers in the store.
c. Why is the interval in (a) narrower than the interval in (b)?