The speed of the boat along the river is 21 km / h. Moving against the current

The speed of the boat along the river is 21 km / h. Moving against the current, it covers 60 km in 4 hours. Find the speed of the river and the speed of the boat?

Let’s find the speed of movement of the boat against the stream of the river in an hour.

60 km / h: 4 hours = 15 km / h.

Let’s find the speed of the river flow, denoting it as an unknown x.

15 + x = 21 – x;

x + x = 21 – 15;

2x = 6;

x = 6: 2;

x = 3.

Let’s find the boat’s own speed, substituting x = 3 instead of the unknown.

15 + 3 = 21 – 3;

18 = 18 km / h.

Answer: the speed of the river is 3 km / h, the own speed of the boat is 18 km / h.