1, about sailing against the current, we must first know two quantitative relations. Downstream speed = boat speed+current speed in still water, and downstream speed = boat speed-current speed in still water. These are two quantitative relations that must be clarified to solve the countercurrent problem.
2. The distance of sailing against the current is constant. Because the distance from pier A to pier B and then from pier B back to pier A is constant. That is, the distance between current and current is constant.
3. Find out the quantitative relationship of the problem: distance downstream = distance upstream. Namely: the speed along the water x the sailing time along the water = the speed against the water x the sailing time against the water.
4. General formula: still water speed (ship speed)+current speed (water speed) = downstream speed; Ship speed-water speed = water flow speed; (downstream speed+upstream speed) ÷2= ship speed; (downstream speed-upstream speed) ÷2= water flow speed. ?
5. Formula for two ships sailing in opposite directions: downstream speed of ship A+upstream speed of ship B = still water speed of ship A+still water speed of ship B; The formula for two ships sailing in the same direction: the still water speed of the rear (front) ship-the still water speed of the front (rear) ship = the speed at which the two ships narrow (expand) the distance.