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Man Crossing a Bridge

A man is walking across a bridge at the rate of 4 mph, when a boat passes under the bridge immediately below him rowing 8 mph. The bridge is 20 feet above the boat. How rapidly are the boat and the pedestrian separating 5 minutes after the boat passes under the bridge?

Byerly, Problems in Differential Calculus, 1895

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"Man Crossing a Bridge," Convergence (February 2008)