They mixed a certain amount of a 13% solution of a certain substance with the same amount of a 43%

They mixed a certain amount of a 13% solution of a certain substance with the same amount of a 43% solution of the same substance. What percentage is the concentration of the resulting solution?

The amount of the first and second solution is the same, which means the total volume doubles.
Take the amount of the substance as x, then the first solution will have a concentration of 0.13x, and the second 0.43x. By combining 2 solutions, we get the amount of substance 0.56x. Since the volume has doubled, the concentration of the solution decreases by 2 times, that is, it will be 56/2 = 28%.