The demand for a certain commodity is

D(x) =500e^-0.2x

units per month when the market price is x dollars per unit.

(a) At what rate is the consumer expenditure E(x) = xD(x) changing with respect to price x when the price is equal to $60 dollars?

(b) At what price does consumer expenditure stop increasing and begin to decrease?

(c) At what price does the rate of consumer expenditure begin to increase?