Tinh can row at a rate of 6 mph in still water. It takes her 2 hours to row upstream from a dock to a park. She then rows back to the dock, and it only takes 40 minutes. What is the speed of the river current?

Let the river current’s speed be c mph. The upstream speed is 6 − c, so the distance to the park is 2(6 − c) miles. The return trip takes 40 minutes, or 2/3 hour, so that same distance is (2/3)(6 + c) miles. Setting the distances equal gives 2(6 − c) = (2/3)(6 + c), which solves to c = 3 mph. The correct choice is A, 3 mph.

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