Figure 10-5 A graph of price, P, versus quantity, Q, shows a supply curve, Private Cost, rising linearly from the origin, a demand curve, Private Value, descending linearly from P = 36, and a second demand curve, Social Value, descending linearly above the first demand curve from P = 63. Curve Private Cost and Curve Private Value intersect at point (240, 24). Curve Private Cost and Curve Social Value intersect at point (420, 42). Point (420, 15) is marked on Curve Private Value. Refer to Figure 10-5. The socially optimal quantity of output is a. 420 units, since the value to the buyer of the 420th unit is equal to the cost incurred by the seller of the 420th unit. b. 240 units, since the value to the buyer of the 240th unit is equal to the cost incurred by the seller of the 240th unit. c. 240 units, since the value to society of the 240th unit is equal to the cost incurred by the seller of the 240th unit. d. 420 units, since the value to society of the 420th unit is equal to the cost incurred by the seller of the 420th unit