The boat swam downstream in 4 hours the same distance that it swims in 8 hours upstream.

The boat swam downstream in 4 hours the same distance that it swims in 8 hours upstream. The speed of the river is 3 km / h. Calculate the speed of the boat in still water.

Solution:
1. Let’s designate: the speed of the boat in still water (own speed of the boat) is equal to x.
2. The boat will sail along the current distance: 4 * (x + 3).
3. The boat will sail against the current distance: 8 * (x – 3).
4. Solve the equation:
4 * (x + 3) = 8 * (x – 3);
4x + 12 = 8x – 24;
8x – 4x = 12 + 24;
4x = 36;
x = 36/4;
x = 9;
The x was the boat’s own speed. It is equal to 9 km / h.
Answer: the speed of the boat in still water is 9 km / h.