Counterfeit coins
Brain Warm-up	Based on My Best Mathematical and Logic Puzzles

Problem:

You are given eight stacks of coins, a pointer scale (one that tells the weight) and the following knowledge. Figure out which stack of coins is counterfeit in the minimal number of weighings and explain the procedure used. You know that all of the coins in one of the stacks are counterfeit. You know that all the coins in the other seven stacks are genuine. You know that each stack has seven coins. You know the exact weight of a genuine coin. You know that each counterfeit coin weighs 1 gram more then a genuine coin.

Solvers:

Pete Lamonica (solved: 1-13-2005 10:23am) tom von kercey (solved: 1-13-2005 1:34pm) The Killer Llama (solved: 1-13-2005 2:03pm)

Solution:

Only 1 weighing is necessary if you, first, set the stacks down in a row and name them stack zero through seven. Then you weigh a single group of coins comprised of 0 coins from stack 0, 1 coin from stack 1, 2 from 2, 3 from 3, and so on. The total extra weight in grams (above 28*[genuine coin weight]) is the counterfeit stacks number.

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