You have 10 bags of gold coins. 9 bags hold 10g coins, but 1 bag holds fake 9g coins. Using a digital scale only once, how can you find the fake bag?
Take 1 coin from bag 1, 2 from bag 2, ..., 10 from bag 10. Weigh them together. Subtract total weight from 550g; the difference in grams is the fake bag number.
If all were genuine, total weight would be (1+2+...+10)*10 = 550g. Each fake coin reduces total weight by 1g.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Hard. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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