Coins Weighing riddle

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There are 20 coin machines, each of which produce the same kind of coin. you know how much a coin is supposed to weigh. One of the machines is defective, in that every coin it produces weighs 1 ounce less than it is supposed to. you also have an electronic weighing machine. how can you determine which of the 20 machines is defective with only one weighing? (by one use, means you put a bunch of stuff on the machine and read a number, and that’s it, you are not allowed to accumulate weight onto the machine and watch the numbers ascend, because that’s just like multiple weighing). you are allowed to crank out as many coins from each machine as you like.

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    You weigh 1 coin from 1st machine, 2 coins from second … 20 coins from the last one. Then you compare the result with a weigh of 210 normal coins. The difference will be the number of machine producing false coins.

    Curious Answered on 30th November 2018.
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