There are 8 marbles that one of them is lighter than the others, and all other seven marbles have the same weight. There is a balance scale that you can use it twice or once.
Is there any way to find the lightest marble? How?
source: Fekraneh.ir
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Put three of the marbles in one bowl of the scale, and put three others in the other bowl.
If the scale is in balance the light marble must be one of the two remaining marbles, so weight those two against one another to find the light marble.
If the scale is not in balance then the light marble must be one of the three marbles from the lightest bowl. Weigh any two of them against one another in order to either find the light marble directly, or conclude that the third is the light marble in case the weighed marbles turn out to weigh the same.
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4$\begingroup$ @JoelRondeau Indeed. I guess that the 8 marble variant might work slightly better as a trick question as people's thoughts tend to get locked on the easy 3 operations solution. $\endgroup$ Sep 2 '14 at 19:39