The boat travels a distance of 40 km downstream in 2 hours. How long does it take to swim

The boat travels a distance of 40 km downstream in 2 hours. How long does it take to swim the same distance upstream? The speed of the river is 5 km / h.

Travel speed:
V = S / t, where V is the speed of movement, S is the distance traveled, t is the time of movement.
Speed of the boat along the river:
V1 = S / t1 = 40/2 = 20 km / h.
Own speed of the boat:
Vк = V1 – Vт, where Vт – river flow speed (Vт = 5 km / h).
Vк = V1 – Vт = 20 – 5 = 15 km / h.
Speed of movement of the boat against the stream of the river:
V2 = Vк – Vт = 15 – 5 = 10 km / h.
Time of boat movement against the current:
t2 = S / V2 = 40/10 = 4 h.
Answer: The boat will sail 40 km against the current in 4 hours.