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Weighty Balls

You have eight balls all of the same size. 7 of them weigh the same, and one of them weighs slightly more. How can you find the ball that is heavier by using a balance and only two weighings?

 

 

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Solution

Weigh 3 balls vs 3 balls.

If one side is heavier, weigh one vs one of those three.  If they are equal, the third is the heavier one (and if not, the heavier is heavier).

If the 3 vs 3 is equal, weigh the two remaining balls and see which is the heaviest!


Difficulty
    • Moderate
      		Type
        • Logic


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            Comments

            Choose 6 balls and weigh 3 against 3
            If they weigh the same, you have another weighing for the remaining 2 balls and you can find the heavier one
            If they don'tt weigh the same, from the group of 3 which was heavier, choose any 2 balls and weigh them:
            If they weigh the same, the remaining ball is the heavier one; otherwise you just found the heavier one by weighing the 2 chosen balls
            - Black Rose (7 May 2009)

            The question doesn't indicate whether how many balls can be used in measuring them so why not putting 4 vs 4, then after finding the heavier set of balls, make a 2 vs 2, then find the heavier set and then 1 vs 1 and see what is the heavier one.
            - Scorpion (Get Over Here!!!) (10 Apr 2010)

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