Mixed a 15% solution of some substance with a 27% solution and got 420 g

Mixed a 15% solution of some substance with a 27% solution and got 420 g of a 25% solution. How many grams of a 15% solution of the substance were taken?

Let’s say that x grams of a 27% solution was taken. Since a total of 420 g of solution was obtained, it means that 420 – x grams of a 15% solution were taken.

According to the condition of the problem, we can compose the following equation:

27 * x / 100 + 15 * (420 – x) / 100 = 420 * 25/100,

27 * x + 15 * 420 – 15 * x = 420 * 25,

12 * x + 6300 = 10500,

12 * x = 10500 – 6300,

12 * x = 4200,

x = 4200: 12,

x = 350 (g) – the mass of a 27% solution.

420 – 350 = 70 (g) – the mass of a 15% solution.

Answer: 70 g.