77. Solving a chemistry problem

3. Determine the composition of the initial mixture, consisting of substances A and B, if the reaction between them goes according to the equation: A, gas + B, gas ↔ C, gas + D, gas. And the equilibrium mixture contains by volume X% of substance C and the same amount of substance D. The values ​​of the equilibrium constant are given in the table.

Task: X=13%, K=4.0

The solution of this task is usually associated with the compilation of 2 equations with two unknown quantities. Therefore, to answer this question, it is necessary to compose and solve a quadratic equation that has 2 possible solutions. Don´t forget to include 2 answers if both make physical sense.