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100 coins. Some of them are fake


You have 10 sets of ten coins. One set of the ten is counterfeit, the others are genuine. The genuine coins weigh exactly 0.10 ounces. The counterfeit coins are exactly a 0.01 ounces off, making the entire set of ten coins 0.10 ounces off. You may use an extremely accurate digital scale only once. How do you determine which set is counterfeit?

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Let's label the sets #0 through #9. Place 0 coins from set #0, 1 coin from set #1,  2 coins from set #2… etc… through 9 coins from set #9 on the scale, all at once. The hundredths digit on the scale display tells you which of the ten sets is counterfeit.


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