A boat covers 12 km in still water in 3 hours. It also covers some distance downstream and the same distance upstream in 9 hours and 3 hours, respectively. What is the speed of the current?

The speed of the boat in still water is 12 km divided by 3 hours, giving 4 km/hr. Let the one-way distance be x km and the speed of the current be y km/hr, so the upstream speed is (4 - y) km/hr and the downstream speed is (4 + y) km/hr. Using the formula time equals distance divided by speed, we have x/(4 - y) = 9 and x/(4 + y) = 3. Dividing the first equation by the second gives (4 + y)/(4 - y) = 3, which solves to 4 + y = 12 - 3y, meaning 4y = 8 and y = 2 km/hr.

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