Wilpen Organization, a price-setting firm, produces nearly 80 %of all tennis balls purchased in the United States. Wilpen estimates the U.S. demand for its tennis balls by using the subsequent linear specification:
Q = a + bP + cM + dPR
where Q is the number of cans of tennis balls sold quarterly, P is the wholesale price Wilpen charges for a can of tennis balls, M is the consumers' average household in- come also PR is the average price of tennis rackets.