Defective stack of coins puzzle


There are 10 stacks of 10 coins each.
Each coin weights 10 gms. However, one stack of coins is defective and each coin in that stack weights only 9 gms.

What is the minimum number of weights you need to take to find which stack is defective? How?

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    SOLUTION-   1 Measurement Only

    We will take,
    1 coin from the first stack,
    2 coins from the second,
    3 from the third
    so on.

    In total we will have 55 coins.
    If all of them were non-defective, they would weigh 550 gms.
    If stack 1 is defective, the measure would read 549 gms.
    If stack 2 is defective, you will read 548 gms.
    and so on.

    So by taking one measurement you can identify, which is the defective stack.

    MrPuzzleFry Genius Answered on 11th September 2015.
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