How much of an alloy that is 20 % copper should be mixed with 400 ounces of an alloy that is 70 % copper in order to get an alloy that is 40 % copper?

You have 400 ounces of 70% copper.How much 20% copper must be mixed in to make 40% copper.
400 * .70 + .20 x = .40 * (400 + x)
280 + .20 x = 160 + .40 x
120 = .20 x
x = 600 ounces of 40% copper.