For a sample of 9 automobiles, the mileage(100s miles) at which the original front brake pads were worn to 9% of their original thickness was measured as was the mileage at which the original rear brake pads were worn to 9% of their orginal thickness. The results were as follows.

car Rear Front

1 41.3 31.0

2 36.2 27.5

3 47.1 38.1

4 46.9 38.7

5 39.4 30.9

6 52.2 42.9

7 52.0 41.8

8 47.0 38.7

9 47.4 38.8

A

99%

confidence interval for the mean difference in lifetime between the front and rear brake pads is