The copper-zinc alloy contained 82% copper. After adding 18kg of zinc to the alloy, the copper content

The copper-zinc alloy contained 82% copper. After adding 18kg of zinc to the alloy, the copper content in the alloy dropped to 70%. How much copper and zinc were in the alloy initially?

We introduce the variable x and denote so the initial mass of the entire alloy.
Let’s write down how much copper and zinc was in it:
x * 82% = 0.82x (kg) – copper;
x * (100% – 82%) = x * 18% = 0.18x (kg) – zinc.
We describe the situation after adding 18 kg of zinc.
x + 18 (kg) is the mass of the entire alloy;
0.18x + 18 (kg) is the mass of zinc.
The percentage of zinc in the new alloy was:
100% – 70% = 30%
(x + 18) * 30% = 0.3x + 5.4 is the mass of zinc.
We get the equation:
0.18x + 18 = 0.3x + 5.4
0.12x = 12.6
x = 105 (kg) – the initial mass of the alloy.
0.82 * 105 = 86.1 (kg) – copper;
0.18 * 105 = 18.9 (kg) – zinc.
Answer: Initially, the alloy contained 86.1 kg of copper and 18.9 kg of zinc.