The boat covered 3 km upstream of the river, and then 28 km downstream, spending 1 hour for the entire journey.

The boat covered 3 km upstream of the river, and then 28 km downstream, spending 1 hour for the entire journey. Find your own speed of the boat if the river speed is 4 km / h.

Let’s enter the designation of the boat’s own speed x km / h, then the speed downstream will be (x + 4) km / h, the speed upstream will be (x – 4) km / h. Distances downstream and upstream are given by condition, we write down the time of movement in both directions:
3 / (x – 4) – time of movement against the current;
28 / (x + 4) – time of movement downstream.
The total travel time was 1 hour. We got the equation:
28 / (x + 4) + 3 / (x – 4) = 1
28x – 112 + 3x + 12 = x² – 16
x² – 31x + 84 = 0
D = 625, x1 = 3, x2 = 28.
Most likely, x = 28 will suit us.
Answer: own speed 28 km / h.