There is an island with 12 islanders. All of the islanders individually weigh exactly the same amount, except for one (either weigh more, or less than the other 11). How can you find which islander is the one that weighs more/less than the others? You must use a see-saw to figure out the weights, and you may only use the see-saw 3 times max
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Put 5 on each side, if the scale doesn't move, then it is 1 of the 2 you DIDN'T put on the scale. If it tips down, then take those 5 and split them between the scale of 2 and 2, and leave one off. If the scale doesn't tip then got it is the one you left off. If the scale tips down, take those 2 and put one on either side.
Problems?
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3$\begingroup$ Doesn't work because you don't know if the odd one out is heavier or lighter. You don't know if you need to take the down or the up side of the see saw $\endgroup$– AifyMar 10, 2015 at 21:20
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$\begingroup$ Put 6 on one side and six on the other. Have one on each side get off until the see saw balances. The last two to get off will have the unequally weighted person. Then take any other person. Seesaw the pair. If balances then the unequally weighted person is the third. Repeat if does not balance. Done. $\endgroup$– ezaspiMar 11, 2015 at 1:59