The average price of large cars getting low gas mileage gas

Question

Question: The average price of large cars getting low gas mileage ("gas guzzlers") is x and the average price of a gallon of gasoline is y. The number, q1, of gas guzzlers bought in a year, depends on both x and y, so q1 = f(x, y). Similarly, if q2 is the number of gallons of gas bought to fill gas guzzlers in a year, then q2 = g(x, y).

(a) What do you expect the signs of ∂q₁/∂x and ∂q₂/∂y to be? Explain.

(b) What do you expect the signs of ∂q₁/∂y and ∂q₂/∂x to be? Explain.