Weighing 10 Bags Puzzle : Bag of Forgery Coins Puzzle - Job Interview Puzzle

Puzzles Details:

You have 10 bags full of coins. In each bag are infinite coins. But one bag is full of forgeries, and you can’t remember which one. But you do know that genuine coins weigh 1 gram, but forgeries weigh 1.1 grams. You have to identify that bag in minimum readings. You are provided with a digital weighing machine.


This "10 Bag of Coins" puzzle is an interesting job interview puzzle. We know that the coins in the forgery bag weigh 1.1 gms each and the coins in the rest of the bags weigh 1 gram each.

  • As there is a random number of coins in each bag, weighing each bag will show different readings. Now just sharpen your analytical skills and narrow down your investigation by taking samples from each bag.
  • If you weigh one sample from each bag, you need to weigh more than once to determine which bag contains the forgery coins, considering that the 1st bag is not a forgery bag.
  • But we can find out the forgery bag in just one time by weighing (reading) on the electronic weighing machine.
  • Take 1 coin from 1st bag, 2 coins from 2nd bag, 3 coins from 3rd bag, so on, and 10 coins from 10th bag.
Now you have a total of 1+2+3+4+5+6+7+8+9+10=55 coins. Weigh all the coins together using the electronic weighing machine. Check the decimals in the reading.

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