An owner of a home in the Midwest installed solar panels to reduce heating costs. After installing the solar panels, he measured the amount of natural gas usedyy( in cubic feet) to heat the home and outside temperaturexx(in degree-days, where a day's degree-days are the number of degrees its average temperature falls below 65oFoF) over a 23-month period. He then computed the least-squares regression line for predictingyfromxand found it to be:

y=y^=82 + 13 x.

How much, on average, does gas used increase for each additional degree-day?

Answer : cubic feet.

(Give your answer as a whole number.)