The motor boat moves against the stream of the river, covers a distance of 36 km in 3 hours.

The motor boat moves against the stream of the river, covers a distance of 36 km in 3 hours. How long will it take this distance back, if the current speed is 2 km / h.

Given:
S = 36 kilometers – the distance a motor boat travels along the river;
t = 3 hours – the time during which the motor boat travels the distance S, moving against the stream of the river;
v1 = 2 kilometers per hour – the speed of the river.
It is required to determine t1 (hour) – how long it takes for the motor boat to cover the distance S, moving back (along the river).
Let’s find the speed with which the boat traveled the distance S against the river flow:
v = S / t = 36/3 = 12 kilometers per hour.
Then the speed of the boat itself will be equal to:
v2 = v + v1 = 12 + 2 = 14 kilometers per hour.
Since the boat will move back along the river, the relative speed of the boat will be:
v3 = v1 + v2 = 14 + 2 = 16 kilometers per hour.
The time will be equal to:
t1 = S / v3 = 36/16 = 2.25 hours.
Answer: the motor boat will cover the way back in 2.25 hours.