pure copper must be added to 50kg of an alloy containing 12% copper to raise the copper content to 21%?
If the percentages are by weight... originally, alloy contains 50kg x 12% = 6 kg copper, 54 kg other final will be 54 kg other, and x copper, to reach 21% x/(x+54) = 0.21 solving for x x = 0.21(x+54) x = 0.21x + 11.34 0.79x = 11.34 x = 14.35 so you need 14.35 kg of copper in the final mix, but you already have 6 kg, so you have to add 14.35 kg – 6 kg = 8.35 kg .
