You have 5 bags filled with coins. Four of the bags contain real gold coins weighing 10 grams each, but one bag contains fake coins weighing 9 grams each. You have a digital scale that provides an exact numeric weight. What is the minimum number of weighings needed to identify the bag with fake coins?
1 weighing
Take 1 coin from Bag 1, 2 coins from Bag 2, 3 from Bag 3, 4 from Bag 4, and 5 from Bag 5 (15 coins total) and place them on the scale together. If all coins were real, the total weight would be 150 grams. Since fake coins are 1 gram lighter, subtracting the measured weight from 150 gives the number of fake coins on the scale, directly pointing to the fake bag (e.g., a weight of 147g means 3 fake coins, so Bag 3 is fake).
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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