Fuel Economy. For Exercises 60–67, refer to the table of fuel economy data from Exercises 80–84 in Section 4.2. The predictor variable is , expressed in liters; the response variable is , expressed in miles per gallon (mpg).
66. Challenge Exercise. Suppose we increased the combined mpg for the Cadillac limousine, so that the slope of the regression line would be exactly zero. What would the combined mpg for the Cadillac limousine have to be to accomplish this?