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Chimera Securities Equity Trader Interview Question (student candidate)

I interviewed in New York, NY and was asked:
"If you have 9 marbles and one is heavier than the other 8. How can you use only a balance beam to figure out which is the heavy one if you can only use it twice."
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Answer

Part of a Equity Trader Interview Review - one of 11 Chimera Securities Interview Reviews

Answers & Comments

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Weigh 3 against 3. If they're the same weight, the heavy one's in the other group of 3.
- Interview Candidate on Jun 7, 2010
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Your answer does not select the actual heaviest marble, nor does it make use of the balance beam twice as indicated in the original problem.

Instead: weigh 4 against 4. IF they are the same weight then the remaining one is the heaviest. OTHERWISE IF one lot is heavier then take the lot of 4 and discard the rest. Then weigh 2 against 2. Take the heavier lot of 2 and place one in each hand. The heavier one is the heaviest marble.
- qq11 on Jul 9, 2010
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Your answer does not select the actual heaviest marble, nor does it make use of the balance beam twice as indicated in the original problem.

Instead: weigh 4 against 4. IF they are the same weight then the remaining one is the heaviest. OTHERWISE IF one lot is heavier then take the lot of 4 and discard the rest. Then weigh 2 against 2. Take the heavier lot of 2 and place one in each hand. The heavier one is the heaviest marble.
- qq11 on Jul 9, 2010
3
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@qq11, I don't think your answer is right either. The question specifically says the balance beam can only be used twice, so I assume the question is asking how to find the heaviest marble by using the beam twice, not 3 times.

You simply just split the 9 marbles into groups of 3. Put two sets on each side of the balance beam, and this will tell you which set has the heavy marble. After that, just take the heavy set of 3 marbles and put 2 of them on the balance beam, and you'll know which marble is the heavy marble.
- Alex on Oct 14, 2010
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@qq : go kill yourself.....are you really thinking about sayng to put the marble in your hand?? c mon bro.......alex is right btw
- alessandro on Jul 12, 2011
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Ooo and ooo and ooo. First try: Ooo vs ooo => Ooo is heavier. Now take two of those and put one on each scale. If equal than the third is heaviest. If not equal, heavier one is heaviest. OR try ooo vs. ooo. Equal. Repeat step two for the pile that is established to be Ooo.
- butthead on Jan 22, 2014

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