Distance to encounter problems = time × speed and

Catch-up distance = catch-up time × speed difference

Sailing in running water is also a common travel problem.

The flowing water problem is a trip problem, so the relationship between the three quantities (speed, time and distance) in the trip problem will be used repeatedly here. In addition, the flowing water problem has the following two basic formulas:

Downstream speed = ship speed+current speed (1)

Current speed = ship speed-water speed (2)

The ship speed here refers to the speed of the ship itself, that is, the distance traveled in still water per unit time. Water speed refers to the distance that water travels in a unit time. Downstream speed and countercurrent speed refer to the distance traveled per unit time when the ship sails downstream and countercurrent respectively.

According to the relationship between the reciprocal operations of addition and subtraction, we can get from formula (L):

Current speed = downstream speed-ship speed,

Ship speed = downstream speed-current speed.

From formula (2), we can get:

Water speed = ship speed-current speed,

Ship speed = current speed+current speed.

That is to say, as long as we know any two of the three quantities: the speed of the ship in still water, the actual speed of the ship and the current speed, we can find the third quantity.

In addition, given the current ship speed and current ship speed, according to formula (1) and formula (2), we can add and subtract them to obtain:

Ship speed = (downstream speed+upstream speed) ÷2,

Water velocity = (downstream velocity-upstream velocity) ÷2.