A boat has a speed of 9 miles per hour in calm water. It takes the boat 4 hours to travel upstream but only 2 hours to travel the same distance downstream. Which equation can be used to find c, the speed of the current in miles per hour?

Upstream, the current slows the boat, so its speed is 9 − c miles per hour. Downstream, the current helps, so its speed is 9 + c miles per hour. The distances are equal, and distance equals speed times time, giving 4(9 − c) = 2(9 + c). This is choice D.

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