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From: rainbow@ihuxe.UUCP
Newsgroups: net.puzzle
Subject: toughest coin weighing puzzle
Message-ID: <644@ihuxe.UUCP>
Date: Wed, 30-May-84 16:41:28 EDT
Article-I.D.: ihuxe.644
Posted: Wed May 30 16:41:28 1984
Date-Received: Wed, 6-Jun-84 05:28:51 EDT
Sender: rainbow@ihuxe.UUCP
Organization: AT&T Bell Labs, Naperville, IL
Lines: 10

Lets forget about these simple cases of 12 and 13 coins. Lets get right into
the toughest one. 26 coins. You have a balance usable three times. You must
determine which one of them is counterfeit and if its lighter or heavier
than the standard. And if that isnt tough enough, lets try 80 coins and
four weighings.. 
  
AS for the solution of 13 coins(the extra coin known to be good is irrelevant  
to the problem), any division of coins works. ie 6-6-1 or 5-5-3 or 4-4-5 or
3-3-7. This is because 7 coins can be solved in two steps which I will leave
to the reader to answer.