How many kilograms of acid do you need to add to 21 liters of water to get a 30% solution?

Suppose the amount of acid to be added to the water is x.
In this case, after adding this amount, the total volume of water will be: 21 + x liters.
To further solve the problem, it is necessary to draw up a proportion in which the amount of acid is 100%, just like the resulting solution is 30%.
In numerical form, we get:
x / (x + 21) * 100% = 30%.
We release from 100%.
x / (x + 21) = 0.3.
x = 0.3 * x + 6.3.
x – 0.3 * x = 6.3
0.7 * x = 6.3.
x = 6.3 / 0.7
x = 9 kg
Answer. You need to add 9 kilograms of acid.