Counterfeit coins  
Brain Warmup  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.

Solvers: 

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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