I mentioned this problem in grade one.

Example:

The distance between a and b is 80 kilometers. A ship sails from A to B for 4 hours, but sails from B to A for 5 hours. What is the sailing speed and current speed of the ship in still water?

Downstream velocity = still water velocity+current velocity

Countercurrent velocity = still water velocity-current velocity

Still water velocity = (downstream velocity+countercurrent velocity) ÷2

Water velocity = (downstream velocity-countercurrent velocity) ÷2

Downward speed: 80÷ 4 = 20km/h.

Countercurrent speed: 80 ÷ 5 =16 km/h.

Still water velocity: (20+16) ÷ 2 =18km/h.

Water velocity: (20-16) ÷ 2 = 2km/h.

It depends on what kind of problem. Something like this.

I used to talk about engineering problems ... it was really troublesome. Later, I naturally did it in junior high school.