The Consumer Surplus is defined as the difference between the maximum price the consumers are willing to pay and the price they actually pay. It is the net gain of the buyers. In mathematical terms, we can derive the consumer surplus as follows. Let p ¼ D1 (q) be the inverse demand function and let p ¼ p0 be the price of the good purchased, i.e., q0 satisfies p0 ¼ D1 (q0), then the Consumer Surplus CS is defined as: CS ¼ Z q0 0 D1 ðqÞdq  p0q0: For a consumer suppose we have q ¼ 50  2p, namely D1ðqÞ ¼ 25  1 2 p. Calculate, according to the above definite integral, the Consumer Surplus at p0 ¼ 20 and q0 ¼ 10.