Suppose that France and Portugal both produce cheese and wine

In this problem, the opportunity cost of one bottle of wine is 2 pounds of cheese for France. The opportunity cost of one bottle of wine is 4 pounds of cheese for Portugal. Since France has to give up less cheese than Portugal to get a bottle of wine, France has a comparative advantage in the production of wine.

To find each country's opportunity cost of producing cheese, we just need to invert their opportunity cost of producing wine. Since the opportunity cost to produce one bottle of wine is 2 pounds of cheese in France, the opportunity cost of producing one pound of cheese in France must be 1/2 bottle of wine. Additionally, since the opportunity cost of producing one bottle of wine is 4 pounds of cheese in Portugal, the opportunity cost of producing one pound of cheese in Portugal must be 1/4 bottles of wine. Portugal has the lower opportunity cost of producing cheese, so it has the comparative advantage in that good.