The motor ship covered 144 km in 6 hours, how long it took for the return trip if the current speed was 4 km / h.

The solution of the problem.
Let’s find the speed of the ship along the river:
144/6 = 24 (km / h) speed of the ship along with the current.
Let’s find our own speed:
24-4 = 20 (km / h) – own speed of the ship.
Back the ship sailed against the current and its speed:
20-4 = 16 (km / h) – speed of the ship against the river flow.
So on the way back he will need
144/16 = 9 (h) – on the way back.
Answer: It took the ship 9 hours for the return journey.