Question

An artist has 30 kg of an alloy that is 45% copper. How many kilograms of a second alloy that is 70% copper should be mixed with the first alloy to get a new alloy that is 60% copper?

Answer 1

Let x be the mass of second alloy, mass of copper in first alloy is 0.45*30 = 13.5 kg.
(x*0.7)+13.5 = 0.6(30+x) , 0.1x = 18 - 13.5 = 4.5.
=> x = 45 kg.
45 kg of new alloy must be mixed.