An alloy weighing 20 lb is 30% copper. How many pounds of another alloy, which is 80% copper, must be added for the final alloy to be 60%?
(20(0.3) + 0.80(x))/(20 + x) = 0.6
So break apart the ingredients. Final % = (final mass of Cu) / (total mass) * 100 Work it out one at a time. The total mass at the end will be 20 + m, if m is the mass of the other alloy we're going to add. And the mass of copper? Well, you've got that 6 lb you started with in the first alloy. To that, you add 0.8m of copper (because 80% of the other alloy is copper). 0.6 = (6 + 0.8m) / (20 + m) 12 + 0.6m = 6 + 0.8m m = ? Best wishes!
