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Bunuel
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GMAT 1: 640 Q49 V29
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Another way to look at the problem:
We are told that the ratio between obsidian and zirconium "gems" is 2x:5x and that the total amount of gems are 36.
We can plug in numbers to find out how many rhinestones would be possible for us to have given the limitations set by the problem.
2(1)+5(1)+r=36 -> r=29
2(2)+5(2)+r=36 -> r=22
2(3)+5(3)+r=36 -> r=15
2(4)+5(4)+r=36 -> r=8
2(5)+5(5)+r=36 -> r=1

As you can see from this list that out of the options given to us, 12 is the only number of rhinestones that isn't possible.
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