9,000 8,000 7,000 Marginal cost 6,000 5,000 Marginal cost and marginal revenue ($) 4,000 E 3,000 2,000 C 1,000 0 Marginal revenue -1,000 Quantity D 0 10 20 30 of cars, Q 40 50 60 70 80 60,000 40,000 Profit ($) 20,000 0 10 20 30 40 50 60 70 80 -20,000 Quantity of cars