A stream flows at a speed of 1 km/h. A motorboat travels 35 km upstream and then returns to the starting point, taking a total of 12 hours. What is the speed of the motorboat in still water?

Explanation

Let the speed of the stream be 1 km/h and the boat's speed in still water be x km/h. The downstream speed is (x + 1) km/h, and the upstream speed is (x - 1) km/h. The total time for the round trip is given by 35/(x + 1) + 35/(x - 1) = 12 hours. Solving this equation, we find x = 6 km/h.