Final answer:
The relationship between Robinson Crusoe and the two ships is that Robinson Crusoe was shipwrecked on a desert island and later joined by a second person named Friday, representing a two-person economy. They engage in fair and voluntary trade to meet their needs, resulting in balanced trade between them.
Step-by-step explanation:
The relationship between Robinson Crusoe and the two ships is that Robinson Crusoe was shipwrecked on a desert island. He lived alone for some time until a second person, named Friday, joined him. Robinson and Friday represent a two-person economy. In this situation, they would engage in trade to meet their needs, such as Robinson trading fish for coconuts with Friday. Each trade would be voluntary and fair, resulting in balanced trade between the two individuals.