The boat went 4 hours against the river and 2 hours along the river, having covered 84 km during all this time

The boat went 4 hours against the river and 2 hours along the river, having covered 84 km during all this time. The boat’s own speed is 15 km / h. Find the speed of the river. Solve the problem using the equation method.

Let’s imagine that the speed of the river flow was equal to a km / h.

Thus, the boat was going downstream at a speed of 15 + a km / h.

Against the current at a speed of 15 – a km / h.

He passed against the stream of the river: 4 * (15 – a) = 60 – 4 * a km.

He walked along the river:

2 * (15 + a) = 30 + 2 * a.

We make an equation for the sum of the distance traveled.

60 – 4 * a + 30 + 2 * a = 84 km.

-2 * a = 84 – 90.

2 * a = 6.

a = 6/2 = 3 km / h.

Examination:

60 – 4 * 3 + 30 + 2 * 3 = 90 – 12 + 6 = 84 km.

Answer:

The speed of the river is 3 km / h.