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Revealing mistake: In the scene at which Dan Aykroyd first joins his employers at the gentleman's club, Don Ameche is shown mirror image when he adds his signature. It happens again at the very end, during Ameche's credit, one of these flipped shots from this scene is repeated. (00:12:00)
Answer: Without knowing precisely how many stocks Winthorp and Valentine sold at the beginning, there is no way to get an exact figure. But we can make some educated guesses. Frozen Concentrated Orange Juice is sold in contracts, with each contract containing 15,000 pounds. By the end of the day, Winthorp and Valentine were making at least an 84 cent profit on each pound, and $12,600 on each contract. If they sold 20,000 contracts, their profit would be $252 million. Considering the Duke's lost $394 million, that estimate seems to fit (and is likely even a little low).