The boat, whose own speed is 15 km / h, covered 36 km upstream and 25 km upstream

The boat, whose own speed is 15 km / h, covered 36 km upstream and 25 km upstream, spending 4 hours for the entire journey. What is the current velocity?

Let us denote the speed of the river as x, the total travel time is 4 hours, the time of the boat on the way downstream is t1, the time of the boat on the way upstream is t2.
4 = t1 + t2;
4 = 36 / (15 + x) + 25 / (15 – x);
36 * (15 – x) + 25 * (15 + x) = 4 * (15 + x) * (15 – x);
36 * (15 – x) + 25 * (15 + x) = 4 * (225 – x ^ 2);
540 – 36 * x + 375 + 25 * x = 900 – 4 * x ^ 2;
15 – 11 * x = – 4 * x ^ 2;
4x ^ 2 – 11 * x + 15 = 0;
When solving a quadratic equation, we get a discriminant less than 0. 11 ^ 2 – 4 * 4 * 15 = 121 – 240 = -119.
The problem has no solution.