A stream flows at 4 km/h. A boat travels 6 km downstream and then returns 6 km upstream, completing the round trip in 2 hours. What is the boat's speed in still water?

Explanation

Let the speed of the boat in still water be x km/h and the speed of the stream be 4 km/h. The downstream speed is (x + 4) km/h, and the upstream speed is (x - 4) km/h. The total time for the trip is given by: 6/(x + 4) + 6/(x - 4) = 2 hours. Solving this equation yields x = 8 km/h.