An alloy is a mixture of metals. Suppose that certain alloy is made by mixing 300 grams of an alloy that contains 72% copper with 20 grams of pure copper. What percentage of the mixture is copper?
The total mass of the mixture is 300 grams + 20 grams = <<300+20=320>>320 grams.
The total mass of copper in the mixture is 300 grams * 0.72 = <<300*0.72=216>>216 grams.
The percentage of copper in the mixture is (216 grams / 320 grams) * 100% = 67.5%. Answer: \boxed{67.5}.