The raft from point A to point B sails 40 hours, and the boat – 4 hours. How long does the boat sail from point B to point A.

1. The distance between points A and B is equal to: S km;

2. The raft sailed from point A to point B in the time: Tp = 40 hours;

3. The speed of the raft is equal to the speed of the river flow: Vp = Vr km / h;

Vр = S / Tп = (S / 40) km / h;

4. The boat sails downstream: Tpo = 4 hours;

Tpo = S / (Vk + Vp) = 4 hours;

Vк + Vр = S / Tpo = S / 4;

Vк = S / Tpo – Vр = S / 4 – S / 40 = S * (9/40) km / h;

5. Consider the movement of the boat against the current:

Tpr = S / (Vк – Vр) = S / (S * (9/40) – (S / 40)) =

S / (S * (9/40 – 1/40)) = 1 / (8/40)) = 5 hours.

Answer: upstream from point B to point A, the boat sails for 5 hours.