by **dbernst** Fri May 04, 2007 10:50 am

To solve this problem quickly, I "broke" several established rules of plugging in numbers. When plugging in numbers on algebraic problems (VICs), it is advisable not to choose 1, 0, or the same number more than once. Doing so will not eliminate the correct answer; however, it will potentially leave you with more than one "correct" answer choice. At first glance, I could not readily find suitable numbers for x,y and z, so I used some GMAT resourcefulness. I knew that the same amount of 1% and 2% milk would provide me with a 1.5% grade, so I chose x=5, y=5, and z=0 (Thus receiving a ruler to the knuckles for breaking "the rules").

However, when I tested each answer choice (solving for x=5, my "target" answer), only choice A gave me the correct value.

Another (slower) option would have been to skip the "plugging in" step altogether, and simply use the answer choices to determine which one would leave me with a 1.5% grade. For example, answer choice a. says y + 3z. If I were to randomly choose numbers for choice a, such as y=2 and z=3, , then x = 11. Now, simply test whether this leaves you with 1.5% milk. With these numbers, I have 11 + 2 + 3 = 16 gallons of milk, and I have 11(1) + 2(2) + 3(3) "parts" fat, = 11 + 4 + 9 = 24. Since 16(1.5) = 24, A is the correct answer.

I will confer with my MGMAT colleagues and one of us will respond with the more advisable way to handle this problem. However, I wanted to post my solution to reinforce the potential to use secondary approaches, even on problems that initially confound you.

-dan