A duck paddles 4 yards downstream in 24 seconds. It takes the duck 40 seconds to paddle the same distance upstream. How fast can the duck paddle in still water ?
4 / 24 =1/6 of a yard/sec. Its speed downstream
4 / 40 =1/10 of a yard/sec. Its speed upstream
[1/6 + 1/10] / 2 =2/15 yards/sec. Its speed in stillwater.
