In 80 coins one coin is counterfiet what is minimum number of weighings to find out counterfiet coin?
by 
Prashant
ANSWERS: 3
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Split in 20,20,20,and 20. Weigh each pile, the one that's different has it. Split 5,5,5, and 5. Weigh each pile, the one different has it. Split 2,2, and 1. Add a good coin to the one to make it a 2 pile. Weigh the 2 piles, the one that is different has it. Weigh a good coin. Weigh one of the 2 left, now you know which one is it. 13 weighs for me--just a guess
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Four weighting are sufficient if we assume that: - we use a balance to compare the piles - the fake coin is lighter than the others Here how it goes: Compare the weight of two piles of 27, the fake coin is in the lighter pile, or in the remaining 26 coins (if equal weight). We have now on pile of 27 (at most). Compare the weight of two piles of 9, the fake coin is in the lighter pile, or in the remaining 9 coins (if equal weight). We have now on pile of 9 (at most). Compare the weight of two piles of 3, the fake coin is in the lighter pile, or in the remaining 3 coins (if equal weight). We have now on pile of 3 (at most). Compare the weight of two coins, the fake coin is the lighter, or the remaining coin (if equal weight). We have now one coin.
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3, if you guess which coin it is in the first three while weighing each coin.
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