12.48 Transportation Research, Inc., has asked you to prepare a multiple regression equation to estimate the effect of variables on fuel economy. The data for this study are contained in the data file Motors, and the dependent variable is miles per gallon—

milpgal—as established by the Department of Transportation certification.

a. Prepare a regression equation that uses vehicle horsepower—horsepower—and vehicle weight—

weight—as independent variables. Determine the predicted value, the confidence interval of the prediction, and the prediction interval when the horsepower is 140 and the vehicle weight is 3,000 pounds.

b. Prepare a second regression equation that adds the number of cylinders—cylinder—as an independent variable to the equation from part

a. Determine the predicted value, the confidence interval of the prediction, and the prediction interval when the horsepower is 140, the number of cylinders is 6 and the vehicle weight is 3,000 pounds.