Thursday, March 9, 2017

Forgeries

You have 10 bags full of coins, in each bag are 1,000 coins. But one bag is full of forgeries, and you can't remember which one. However, you know that a genuine coin weigh 1 gram, but forgeries weigh 1.1 grams. To hide the fact that you can't remember which bag contains forgeries, you plan to go just once to the central weighing machine to get one accurate weight. How can you identify the bag with the forgeries with only one weighing?

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Answer:

Take 1 coin from the first bag, 2 coins from the second bag, . . ., and 10 coins from the tenth bag and weigh the picked coins together! If there were no forgeries, you know that the total weight should be (1+2+3+ . . . +10) = 55 grams. However, as an example, if the weight is 55.3 grams, then you know that 3 coins are forgeries, so that must be the third bag.





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