Cellulon, a manufacturer of home insulation, wants to develop guidelines for builders and consumers on how the thickness of the insulation in the attic of a home and the outdoor temperature affect natural gas consumption. In the laboratory, it varied the insulation thickness and temperature. A few of the findings are:


Monthly Natural Gas Consumption (cubic feet),
y Thickness of Insulation (inches),
x1 Outdoor Temperature (ºF),
x2
30.3 4.0 42.0
24.8 12 40
31.7 8 49

On the basis of the sample results, the regression equation is:

yˆ = 62.93 − 1.25x1 − 0.78x2

a. How much natural gas can homeowners expect to use per month if they install 4 inches of insulation and the outdoor temperature is 42 degrees F? (Round your answer to 2 decimal places.)



b. What effect would installing 7 inches of insulation instead of 4 have on the monthly natural gas consumption (assuming the outdoor temperature remains at 42 degrees F)? (Round your answers to 2 decimal places.)



c. Why are the regression coefficients b1 and b2 negative? Is this logical?