Suppose that Spain and Germany both produce jeans and shoes. Spain's opportunity cost of producing a pair of shoes is 3 pairs of jeans while Germany's opportunity cost of producing a pair of shoes is 11 pairs of jeans. By comparing the opportunity cost of producing shoes in the two countries, you can tell that has a comparative advantage in the production of shoes and has a comparative advantage in the production of jeans.
Suppose that Spain and Germany consider trading shoes and jeans with each other. Spain can gain from specialization and trade as long as it receives more than of jeans for each pair of shoes it exports to Germany. Similarly, Germany can gain from trade as long as it receives more than of shoes for each pair of jeans it exports to Spain.
Based on your answer to the last question, which of the following prices of trade (that is, price of shoes in terms of jeans) would allow both Germany and Spain to gain from trade?
4 pairs of jeans per pair of shoes, 1 pair of jeans per pair of shoes, 6 pairs of jeans per pair of shoes, 2 pairs of jeans per pair of shoes

By comparing the opportunity cost of producing shoes in the two countries, you can tell that SPAIN has a comparative advantage in the production of shoes and GERMANY has a comparative advantage in the production of jeans.

Suppose that Spain and Germany consider trading shoes and jeans with each other. Spain can gain from specialization and trade as long as it receives more than 3 PAIRS of jeans for each pair of shoes it exports to Germany. Similarly, Germany can gain from trade as long as it receives more than ¹/₁₁ PAIR of shoes for each pair of jeans it exports to Spain.

Based on your answer to the last question, which of the following prices of trade (that is, price of shoes in terms of jeans) would allow both Germany and Spain to gain from trade?

Explanation:

Opportunity costs refer to the extra costs or benefits lost resulting from choosing one investment or activity over another alternative. In this case, if Spain specializes in the production of shoes, it will not produce jeans anymore. The opposite would happen to Germany.

you have to put the parentheses like this:

in fact, you have to perform the operation inside parentheses first, and then proceed in order: