Gas mileage estimates for cars and light-duty trucks are determined and published by the U.S. Environmental Protection Agency (EPA). According to the EPA, ". . . the mileages obtained by most drivers will be within plus or minus 15 percent of the [EPA] estimates. . . ." Themileage estimate given for one model is 23 mpg on the highway. If the EPAclaim is true, the standard deviation of mileages should be about 0.15 · 23/3 = 1.15 mpg. A random sample of 12 cars of this model yields the following highway mileages.

a. At the 5% significance level, do the data suggest that the standard deviation of highway mileages for all cars of this model is different from 1.15 mpg?
b. Why it is useful to know the standard deviation of the gas mileages as well as the mean gas mileage?

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