Question

How many ounces of pure zinc must be alloyed with 20 ounces of brass that is 25% zinc and 75% copper to produce brass that is 50% zinc?

Answer 1

20 ounces of brass that is 25% zinc and 75% copper contains
25% of 20 oz of zinc and
75% of 20 oz of copper
That means the 20 oz of brass have
5 oz of zinc and
15 oz of copper.

Since you want he final brass to be 50% zinc, you need the same amount of zinc and copper. How much zinc do you need to add to 5 oz of zinc to end up with 15 oz of zinc in the brass?