一道顺逆流问题If a boat leaves dock A for dock B in downstream dire
一道顺逆流问题
If a boat leaves dock A for dock B in downstream direction,it takes 4 hours;and if the boat leaves dock B for dock A in countercurrent direction,it takes 5 hours.We know that the water-flow speed is 3km/h,please calculate the speed of boat travelling in still water.(并翻译)