8000: (15 икс+4 + 13 икс) - 15=25

8000 / (15x + 4 + 13x) - 15 = 25;

8000 / (15x + 4 + 13x) = 25 + 15;

8000 / (15x + 4 + 13x) = 40;

8000 = 40 * (15x + 4 + 13x);

8000 / 40 = 15x + 4 + 13x;

15x + 4 + 13x = 200;

15x + 13x = 200 - 4;

15x + 13x = 196;

28x = 196;

x = 196 / 28;

x = 7.

Ответ : x = 7.