This is only for people with really good logical skills.
There are 10 bags each with 100 gold coins in them.
They all contain identical looking coins which weigh 1 gram each, except for one bag of counterfeit coins which weigh 1.1 grams each.
You have an accurate scale but you are only allowed to weigh once.
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Number the bags from 1 to 10.
Take 1 coin from the 1st bag, 2 from the 2nd, 3 from the 3rd and so on, until the 9th bag. So the number of coins will correspond to the number on the bag.
You will have 45 coins with you, weigh these coins.
The weight will be 45 point something and the number after the decimal point will correspond with the number of bag.
i.e. If the weighed weight is 45.5 grams, it would mean the 5th bag contains the counterfeit coins
If the coins weigh exactly 45 grams then you know the 10th bag (which didn’t get weighed) contains the counterfeit coins.