The boat travels 10 km downstream in 4 hours in less than 6 hours upstream.

The boat travels 10 km downstream in 4 hours in less than 6 hours upstream. Find your own speed of the boat if the raft on this river in 15 hours sails the same distance as the boat in 2 hours on the lake.

1. Take the boat’s own speed as x.

2. In 2 hours the boat will sail 2 kilometers along the lake. The raft will sail along the river in 15 hours also 2 kilometers. In 1 hour the raft will sail along the river 2x / 15. This is the speed of the river.

3. Speed of the boat along the river (х + 2х / 15).

Speed of the boat against the stream of the river (x – 2x / 15).

4. We compose and solve the equation:

6 (x – 2x / 15) – 4 (x + 2x / 15) = 10;

6 (13x) / 15 – 4 (17x) / 15 = 10;

10x / 15 = 10;

x = 15.

Answer: the boat’s own speed is 15 km / h.