The Environmental Protection Agency (EPA) conducted an experiment to determine the effect of adding ethanol to gasoline. First 40 automobiles had their gas tanks completely drained and then filled with exactly 15 gallons of gasoline without ethanol. Then these auto were driven on a closed course for 200 miles. Next their gas tanks were drained and the amount of gasoline used by each auto was thus determined. The second phase of the experiment was to fill these same automobiles with 15 gallons of gasoline with ethanol, drive them on the same closed course for 200 miles, and then drain their gas tanks to determine the amount of fuel used by each auto. The differences between the gasoline without and with ethanol were computed for each automobile, and the EPA computed the mean of these differences as 0.31 gallons and a standard deviation of 0.74 . Let ? = 0.01 , and test to see if there is a significant difference in the mean number of gallons used between the two types of gasoline.