So you use "x" as the amount of 100% alloy you're going to put in, and then "132-x" as the amount of 12% alloy you're going to put in (because total there's 132 g).
Convert the % into decimals... you get:
1x + 0.12(132-x) = 0.2(132). The right side b/c there are 132 g of 0.2% copper & the left side should make 132 g of 0.2% copper, so the equations are equal.
x + 15.84 - 0.12x = 26.4
0.88x + 15.84 = 26.4
0.88x = 10.56
x = 12 g
You would need 12 g of 100% alloy, and 110 g of 12% alloy to make 132 g of 20% alloy.