Hi All,
These types of "weighted" average questions can be solved in a few different ways. You can use the Weighted Average formula, you can use Allegation (which is a ratio-based approach) and (at least in this question) you can TEST THE ANSWERS....
We're told that 10g of water are mixed with an unknown amount of 30% barium mixture to form a 20% barium final mixture. We're asked to figure out how much barium mixture we have....
Logically, if we had 10g of water mixed with 10g of barium mixture, we'd end up with a 15% overall mixture, so 10 is NOT the answer. We need MORE barium mixture to raise the final mixture up to 20%. Since the numbers in the prompt are all "round" numbers, it's highly likely that the correct answer will ALSO be a round number....
Let's TEST Answer C = 20g
(20g of barium mixture)(30% barium) = 6g barium
+ 10g water.....
(6g barium)/(30g total liquid) = .2 = 20% final barium mixture.
So, the initial barium mixture MUST be 20g
Final Answer:
GMAT assassins aren't born, they're made,
Rich
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