Alloy weighing 800 grams contains 15 percent midi. Copper sludge must be added to this alloy to make 20 percent copper.

Let’s find how much copper is in 800 grams of the original alloy:
800 * 15/100 = 120 g.
We take the mass of copper to be added to the alloy as x. Then the alloy will weigh (800 + x) g.
And the copper content in it will be (120 + x)
Let’s make an equation based on the condition that the new alloy will contain 20% copper:
(800 + x) * 0.2 = 120 + x;
0.8 * x = 160 – 120;
0.8 * x = 40;
x = 50 g.
Answer: to get a new 20% alloy, you need to add 50 grams of copper.