Final answer:
The two halves of a barrel metaphorically represent an equivalent exchange in trade, ensuring each party benefits from the transaction. For the U.S. to profit from trading corn for oil with Saudi Arabia, it should receive more than a half barrel of oil for each bushel of corn.
Step-by-step explanation:
From an economic perspective, the duration for which the two halves of a barrel need to stay together is not a matter of time, but a matter of ensuring equivalent exchange in trade. When the United States is engaging in trade with Saudi Arabia, for example, the transaction should be mutually beneficial. If the U.S. is exporting 100 bushels of corn, to maintain equivalence in value, it needs to import at least 50 barrels of oil, since one barrel of oil is equal to two bushels of corn. To ensure that the trade is advantageous, the U.S. should aim to receive more than a half barrel of oil per bushel of corn exported.
If we consider the concept of gains from trade, it's not just about meeting an equivalence but about obtaining a surplus benefit from the exchange. This principle is why the U.S. would need to gain more than a half barrel for each bushel of corn to justify the trade with Saudi Arabia. Therefore, the duration of unity between the two halves of a barrel, or the balance of trade, should be maintained until a more profitable trade can be negotiated.