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How do I solve the coin problem?

Can anyone help me with this coin puzzle ?

  • Here is the puzzle,and I want an answer for it - I have 12 coins.One of them is fake,but the fake coin has the same appearance as of the real coins but has different weight.We have to find out the fake coin through the shortest number of measurements with a weighing balance. So can anyone solve this problem ? And there are also choices - (A) 2 (B) 2 (C) 4 (D) 6 (My teacher told me that this would not be the answer after I asked him to give me a hint.)

  • Answer:

    the answer would be 3 or if there were 9 balls the ans would have been 2

SomePers... at Yahoo! Answers Visit the source

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Niko's method only works if you know in advance whether the fake coin is heavier or lighter than the others, which you don't in this case. Manan's answer is correct; refer to his source for a complete explanation.

Chris

Get the 12 and split them in half (6 coins in each pile). Measure the weight of both piles. Keep the pile with the different weight (does this coin weigh more or less?) and get rid of the other pile. That pile of 6 coins, now split that one into two groups with each pile having 3 coins. Once again, measure both piles and get rid of the pile that has all coins as normal. Then you have 3 coins left. This one just pick two and measure one against one. If both coins that you picked weigh the same, then the fake coin is the leftover of the three. If there is a coin difference, then you just found your fake coin. All in all, this should take three tries. Though you wrote 2 twice up there as the answer choices. Hope this helps.

Niko

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