The Following Regression Model Has Been Proposed to Predict Sales $\hat { Y }$

The following regression model has been proposed to predict sales at a fast food outlet.
$\hat { Y }$ =18 - 2X1+7X2+15X3
where
X1 = the number of competitors within 1 mile X2 = the population within 1 mile
X3 = 1 if drive-up windows are present, 0 if otherwise
$\hat { Y }$ = sales in $1,000s)
a. What is the interpretation of 15 the coefficient of X3) in the regression equation?
b. Predict sales for a store with 2 competitors, a population of 10,000 within one mile, and one drive-up window give the answer in dollars).
c. Predict sales for the store with 2 competitors, a population of 10,000 within one mile, and no drive-up window give the answer in dollars).