Defective stack of coins puzzle
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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?
SOLUTION- 1 Measurement Only
EXPLANATION-
We will take,
1 coin from the first stack,
2 coins from the second,
3 from the third
and
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.
Your Answer
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