The speed of the boat in still water is 21 km / h and along the river 25 km / h. Find the speed of the boat upstream.

To solve this task, you first need to enter the rate of movement of the river beyond the unknown x, then you need to add the rate of movement of the boat in still water to the rate of movement of the river:

1) 21 + x = 25;

x = 25 – 21;

x = 4 kilometers per hour the rate of movement of the river.
Now we have the opportunity to find the speed of the boat moving behind the current, for this we need to find the difference between the speed of the boat in still water and the speed of the river:

2) 21 – 4 = 17 kilometers per hour.
Answer: the pace of movement of the boat against the current is 17 kilometers per hour.