A “black market” is a place where people make illegal trades in goods and services. For instance, during the Soviet era, it was common for American tourists to take a few extra pairs of Levi’s jeans when visiting the Soviet Union: They would sell the extra pairs at high prices on the illegal black market. 

Consider the following claim: “Price-controlled markets tend to create black markets.” Let’s illustrate with the following figure. If there is a price ceiling in the market for cancer medication of $50 per pill, what is the widest price range within which you can definitely find both a buyer and a seller who would be willing to il­legally exchange a pill for money?