An independent mail delivery service wants to study factors that affect the daily gas usage of its delivery trucks. Using data collected from different trucks on various days, a company analyst uses software to fit a regression model of the following form. y = -7.5 +8x1 -x2+ 4x3+0.01x4 In this model are the following variables. y= volume of gasoline used (in gallons) x = weight of truck (in tons) x, = tire pressure (in psi, pounds per square inch) x2 = weight of initial package load (in hundreds of pounds) x = total distance driven while delivering packages (in miles) Answer the questions below for the interpretation of the coefficient of x, in this model. (a) Holding the other variables fixed, what is the average change in daily fuel used for each additional pound of air pressure (per square inch) used in a truck's tires? X ? gallon (s) (b) Is this change an increase or a decrease? Increase decrease