工人在工厂工作。

Workers work in the factories.

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50年前，1460万人，即28%的美国工人在工厂工作。

Fifty years ago, 14.6 million people, or 28 percent of all U.S. workers, were employed in factories.

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我们也知道，这涉及到每个工人在工厂工作的某一天，只有一次进入工厂和叶子在这一天恰好一次。

We also know that each worker that comes to work in the factory on a certain day, enters the factory exactly once and leaves it exactly once on that day.

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