The alloy weighing 2 kg contains 75% copper. How much zinc do you need to add to make the new alloy 60% copper?

You need to convert the percentage to a fraction: 75% = 0.75.
In this copper alloy, it turns out to be 0.75 * 2 = 1.5 kg.
Let it be necessary to add x kg of zinc, then copper in the new alloy will be 0.6 * (x + 2) kg.
It is necessary to solve the equality: 0.6 * (x + 2) = 1.5;
0.6 * x + 0.6 * 2 = 1.5;
0.6 * x +1.2 = 1.5;
0.6 * x = 1.5 – 1.2;
0.6 * x = 0.3;
x = 0.3 / 0.6;
x = 0.5.
Answer: If you add 0.5 kg of zinc, then the copper alloy will be 60%.