In the x quarts of 2% fat milk, there is 0.02x quarts of fat. in the y quarts of 4% fat milk there is 0.04y quarts of fat. together, there is (x+y) quarts of milk, and 0.24 quarts of fat. the equations are: x+y=9 0.02x+0.04y=0.24 multiply the first equation with 0.02: 0.02x+0.02y=0.18 subtract this new equation from equaition 2 to eliminate x: 0.04y-0.02y=0.24-0.18 0.02y=0.06 y=3 x=9-3=6 x is twice as much as y. He used twice as much as the 2% fat milk than the 4% fat milk. so the first choice is the correct answer.
