Tin is 10% bronze, the rest is copper. How many kg of copper do you need to take to get 28 kg of bronze?

The entire alloy of bronze is 100%, and tin in it is 10%, which means copper in bronze is 90%.
1) 100% – 10% = 90 (%) copper contains bronze.
The entire alloy of bronze is 100%, copper in this alloy is 90%, which means that copper takes up 0.9 part of the entire alloy.
2) 90% / 100% = 0.9 (part) of bronze is occupied by copper.
The bronze alloy needs to be 28 kg, and copper takes up 0.9 part of the bronze alloy, which means that copper will need 25.2 kg.
3) 28 * 0.9 = 25.2 (copper) is needed to get 28 kg of bronze.
Answer: copper will need 25.2 kg.