suppose the economy initially produces 12 million gallons of drinking water and 500,000 tons of coal, which is represented by point a. the opportunity cost of producing an additional 4 million gallons of drinking water (that is, moving production to point b) is80,000 tons of coal. suppose, instead, that the economy currently produces 420,000 tons of coal and 16 million gallons of drinking water, which is represented by point b. now the opportunity cost of producing an additional 4 million gallons of drinking water (that is, moving to point c) is tons of coal.