4.35 Saving energy with solar panels. We have data w from a house in the Midwest that uses natural gas for heat ing. Will installing solar panels reduce the amount of gas con sumed? Gas consumption is higher in cold weather, so the rela tionship between outside temperature and gas consumption is important. Here are data for 16 consecutive months:"

Outside temperature is recorded in degree-days, a common measure of demand for heating. A day's degree-days are the number of degrees its average temperature falls below 65°F. Gas used is recorded in hundreds of cubic feet. The chart gives data for 23 more months after installing solar panels.

What do the before-and-after data show about the effect of solar panels? (Start by plotting both sets of data on the same plot, using two different plotting symbols.)

Nov. Dec. Jan. Feb. Mar. Apr. May June Degree-days per day 24 51 43 33 26 13 4 0 Gas used per day 6.3 10.9 8.9 7.5 5.3 4.0 1.7 1.2 July Aug. Sept. Oct. Nov. Dec. Jan. Feb. Degree-days per day Gas used per day 0 1 6 12 30 32 52 30 1.2 1.2 2.1 3.1 6.4 7.2 11.0 6.9