how much cheese mixture is left to cook with the macaroni?

Question

a recipe for homemade macaroni and cheese calls for a mixture of 2/3 cup of american cheese and a 1/2 cup of cheddar cheese. after mixing you need to save 1/4 cup of mixture to sprinkle on the top.

Answer 1

2/3c + 1/2c - 1/4c = x

multiply each element by 12, giving

8 + 6 - 3 = 12x

solving, x = 11/12c or 0.917c