Hi All,

This is a 'Weighted Average' question, so it can be solved in a number of different ways. For most Test Takers, setting up the Weighted Average formula and doing the necessary algebra is a straight-forward way to get to the solution, but there ARE alternatives. Since the question asks for the number of milliliters of Solution Y that are needed, and the answer choices are NUMBERS, we can TEST THE ANSWERS.

From the prompt, we are told to mix 200 milliliters of a 10% alcohol solution with an unknown number of milliliters of a 30% alcohol solution to get a 25% total alcohol solution.

IF....we mixed 200 milliliters with another 200 milliliters, then we'd end up with a 20% mixture (since the average of 10% and 30% is 20%). That result would be TOO LOW, so we clearly need a lot MORE milliliters of the 30% solution. From this, we can eliminate Answers A and B from consideration.

Let's TEST Answer D....

IF...we have 480 milliliters of Solution Y, we have....

Does [.1(200) + .3(480)]/680 = .25?

Does 20 + 144 = .25(680)?

Does 164 = 170?

No it doesn't, so Answer D is NOT the answer. We likely need to add MORE of the 30% solution to get the two 'sides' to match, but we can confirm this by TESTing Answer E.

IF....we have 600 milliliters of Solution Y, we have...

Does [.1(200) + .3(600)]/800 = .25?

Does 20 + 180 = .25(800)?

Does 200 = 200?

Yes it does, so this MUST be the answer.

Final Answer:

GMAT assassins aren't born, they're made,

Rich

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