To increase the quantity of berries from 21 pounds to 26 pounds (from row F to row E of Table 1 ), production of fish decreases from 15 pounds to 13 pounds. To gain 5 pounds of berries, Crusoe must forgo 2 pounds of fish. The opportunity cost of 1 pound of berries is the 2 pounds of fish forgone divided by 5 pounds of berries gained—2/5 of a pound of fish.