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 Two false coins (Posted on 2005-03-25)
You have ten coins, but two are fake, and weigh a little less. How many times do you have to use a two arm scale, in order to pick out the two fakes?

 Submitted by Federico Kereki No Rating Solution: (Hide) There are (10,2)=45 ways to pick the two coins out of the ten. Each use of the scale gives three possible result. Thus, with less than 4 uses, you cannot distinguish among 45 cases. Now, you need an algorithm that does this, to complete the proof, and there are such solutions among the comments...Extra note: if the fake coins didn't weigh the same, we would have 90 ways to pick the two coins, so the scale would have to be used 5 times. Once again, this is a minimum, and until you find the algorithm, you cannot say it's the actual minimum.

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 Subject Author Date Answer K Sengupta 2009-01-09 12:41:16 Solution without knowing if the fakes are equal Brian Smith 2005-06-02 21:18:14 hmmmmmmm Taylor 2005-04-09 23:41:08 re: Existential doubt john 2005-04-06 16:53:47 re: Guess David 2005-03-28 05:37:24 Guess David 2005-03-28 05:33:41 Existential doubt Federico Kereki 2005-03-27 22:29:30 Solution Eric 2005-03-26 18:31:13 Possibility Kardo 2005-03-26 07:31:28 re: thoughts pete 2005-03-26 05:25:35 thoughts Charlie 2005-03-25 20:30:57

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