Haji rows his canoe 10 mph in still water. He noticed one day that he could row 4 miles upstream in the same amount of time he could row 6 miles downstream. What was the approximate speed of the current that day?

Let c be the current’s speed. The upstream and downstream speeds are 10 − c and 10 + c, so equal travel times give 4/(10 − c) = 6/(10 + c). Solving gives 40 + 4c = 60 − 6c, hence c = 2 mph; the current was approximately 2.0 mph, choice A.

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