Assume absence of trade under increasing opportunity costs, US producing wheat 18 (y-axis) and autos 5 (x-axis). The relative cost of wheat into autos (slope of the line) is .33 bushels of wheat. So, to produce one auto, the nation needs to give up .33 bushels of wheat? How the book came up with this slope? Why is not 18/5= 3.6 the slope?