How many cups of grape punch containing 10% fruit juice and berry punch containing 20% fruit juice must be added together to create 12 cups of punch with 18% fruit juice?

Well, let's say we need "g" of grape punch, now how much juice is in the "g" amount? well, just 10% of it is juice or (10/100) * g, 0.1g.

let's say we need "b" of berry punch, now, is 20% juice, how much juice in "b"? well, (20/100) * b, or 0.2b.

whatever "g" and "b" are, they must add up to 12 cups, of 18%, how much juice in 12cups? well (18/100) * 12, or 2.16





and of berry will then be 12 - g