The demand for a particular commodity is given by D(x) = −35x + 200; that is, x units will be sold (demanded) at a price of p = D(x) dollars per unit.

a. Consumer expenditure E(x) is the total amount of money consumers pay to buy x units. Express consumer expenditure E as a function of x.

b. Find the average rate of change in consumer expenditure as x changes from x = 4 to x = 5.

c. Use calculus to find the instantaneous rate of change of expenditure with respect to x when x = 4. Is expenditure increasing or decreasing when x = 4?