The motor boat, whose own speed is 8 km / h, traveled along the river a distance of 15 km downstream

The motor boat, whose own speed is 8 km / h, traveled along the river a distance of 15 km downstream and the same distance upstream. Find the speed of the river flow if the time taken for the entire journey is 4 hours.

1) Let the flow velocity be equal to x;
Then the speed of the boat along the river will be: 8 + x;
the speed of the boat against the stream of the river will be: 8 – x;
2) The equation of motion of the boat:
15 / (8 + x) + 15 / (8 – x) = 4;
15 (8 – x) + 15 (8 + x) = 4 (8 – x) (8 + x);
120 – 15x + 120 + 15x = 4 (64 – x ^ 2);
4x ^ 2 = 16;
x1 = 2;
x2 = -2 does not meet the conditions;
Answer: The speed of the river is 2 km / h;