On the river, the distance between the two berths is 36 km. How much time will a motorboat spend

On the river, the distance between the two berths is 36 km. How much time will a motorboat spend on the way from one berth to another and back, if its speed is 10 km / h, and the speed of the river is 3 km / h?

Since the boat will sail in two directions, we find the speed of the boat both with the current and against:
10 + 3 = 13 (km / h);
10 – 3 = 7 (km / h).
We find the time of movement of the boat:
36/13 + 36/7 = 2.769 + 5.142 = 7.911 ≈ 8 (hours).
The answer turned out to be inaccurate, possibly a typo in the condition.
Answer: the boat will spend 8 hours on the way back and forth.