Find the fake coin

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Topic: Find the fake coin (Read 5900 times)

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Find the fake coin

« on: October 11, 2007, 10:06:20 am »

You have nine visually equal coins, but one of them is fake and it is a little bit lighter than the others. You have a precision scales, but you do not want to loose many time and you preffer to find the fake coin with only two weighs. Please describe how you can find the fake coin with only two weighs...


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Re: Find the fake coin

« Reply #1 on: October 11, 2007, 11:30:27 am »

1. Arrange all coins in groups of threes.
2. Pick any two groups and put them on the scales.
3a. If scales show that these groups are equal then pick any two coins from group whitch was left and put them on the scales. If one coin is lighter than another then it is the fake coin. If they are equal, fake coin is the coin left unscaled.

3b. ELSE If scales show that one group is lighter than another, then pick any two coins from lighter group and put them on scales. If one coin is lighter than another then it is the fake coin. If they are equal, fake coin is the coin left unscaled.