The motor boat moves along the river from point A to point B for 4 hours, and back – 5 hours.

The motor boat moves along the river from point A to point B for 4 hours, and back – 5 hours. What is the speed of the river if the distance between points is 80 km?

S = (V + V1) * t1, S is the distance between points A and B (S = 80 km), V is the motor boat’s own speed (km / h), V1 is the river flow speed (km / h), t1 is time of movement of the motor boat (t = 4 h).

S = (V – V1) * t2, t2 – time of movement of the motor boat (t = 5 hours).

(V + V1) * t1 = (V – V1) * t2.

4V + 4V1 = 5V – 5V1.

V = 9V1.

Let’s calculate the speed of the current:

S = (9V1 + V1) * t1.

80 = 10V1 * 4.

V1 = 80/40 = 2 km / h.

Examination:

S = (9V1 – V1) * t2.

80 = 8V1 * 5.

V1 = 80/40 = 2 km / h.