You have 10 stacks of 10 coins each. One entire stack consists of counterfeit coins, while the other 9 stacks contain genuine coins. A genuine coin weighs 10 grams, but a counterfeit coin weighs 11 grams. You have a digital scale that provides an exact numerical weight in grams. What is the minimum number of weighings required to identify the counterfeit stack?
1
Take 1 coin from stack 1, 2 coins from stack 2, 3 coins from stack 3, and so on up to 10 coins from stack 10. Weigh all 55 coins together. If all were genuine, the weight would be 550 grams. Subtract 550 from the actual scale reading; the remainder tells you the stack number (e.g., an excess of 4 grams means stack 4 is counterfeit).
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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