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）
问题描述：
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）
答
一艘船离开码头A到B在下游码头方向,但需要4个小时;假如船停靠码头B到A在逆流方向,它需要5个小时.我们知道水流速3公里/小时的速度,请计算船旅行在静止的水时的速度.
设x,距离相等
4（x+3）=5（x-3）
x=27