Refer to Exercise 7. In a separate experiment, 18 cars were outfitted with tires with tread type A, and another 18 were outfitted with tires with tread type B.

Each car was driven 1000 miles. The cars with tread type A averaged 23.93 mpg, with a standard deviation of 1.79 mpg. The cars with tread type B averaged 22.19 mpg, with a standard deviation of 1.95 mpg.

a. Which method should be used to find a confidence interval for the difference between the mean mileages of the two tread types: expression (7.14) (in this section) or expression (7.11) (in Section 7.3)?

b. Using the appropriate method, find a 99% confidence interval for the difference between the mean mileages of the two tread types.

c. Is the confidence interval found in part

(b) wider than the one found in Exercise 7a? Why is this so?