In an alloy of copper and tin, the mass of copper was reduced by 25%, and the mass of tin was increased by 15%.

In an alloy of copper and tin, the mass of copper was reduced by 25%, and the mass of tin was increased by 15%. As a result, the alloy began to weigh 20% less than the original. How many% was the mass of tin from the original alloy?

1. The mass of copper in the original alloy: Mm kg;

2. Mass of tin in the original alloy: Mo kg;

3. The mass of copper was reduced by: Km = 25%;

4. The mass of tin was increased by: Ko = 15%;

5. The initial alloy began to weigh less by: Ku = 20%;

6. We compose the balance for copper and tin in the original and obtained alloy:

(Mm + Mo) – 0.25 * Mm + 0.15 * Mo = (Mm + Mo) – 0.2 * (Mm + Mo);

0.15 * Mo + 0.2 * Mo = 0.25 * Mm – 0.2 * Mm;

0.35 * Mo = 0.05 * Mm;

Mm = (0.35 / 0.05) * Mo = 7 * Mo;

6. The mass of the initial alloy: M = Mo + Mm = 7 * Mo + Mo = 8 * Mo;

7. Tin content: K = Mo / M * 100 = Mo / (8 * Mo) * 100 = 12.5%.

Answer: the original alloy contains 12.5% tin.