The boat went 80 km along the river and returned back, spending 9 hours

The boat went 80 km along the river and returned back, spending 9 hours for the whole journey. find the speed of the river if the speed of the cabyer in still water equals 18 km / h.

We draw up an equation in which the speed of the river flow is written as x km / h.
Thus, the speed of the boat along the river will be equal to: 18 + х km / h.
The speed of the boat upstream of the river will be: 18 km / h.
We get the equation for the amount of time spent.
80 / (18 + x) + 80 / (18 – x) = 9.
80 * (18 – x) + 80 * (18 + x) = 9 * (324 – 18 * x + 18 * x – x^2).
1440 – 80 * x + 1440 + 80 * x = 2916 – x2.
x^2 – 2916 + 2880 = 0.
x^2 = 36.
x = √36 = 6 km / h.
Answer:

The speed of the river is 6 km / h.