A motorboat covers 12 km upstream and returns back to the starting jetty in 3 hours. If the speed of the boat in still water is 9 km/h, find the speed of the river current.
Explanation:
Let the speed of the current be c km/h. 12 / (9 - c) + 12 / (9 + c) = 3. Dividing by 3: 4 / (9 - c) + 4 / (9 + c) = 1 => 4(9 + c + 9 - c) = 81 - c^2 => 72 = 81 - c^2 => c^2 = 9 => c = 3 km/h.
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