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Summer Maths Puzzles - Friday 9 August - Counterfeit Conundrum

Counterfeit Conundrum

Following their seaside escapades, the youngest of the children is now obsessed with all things pirate-related.  Inspired by this, the parents go to a local sweet shop and buy a bag of 12 chocolate coins and set the children the following challenge: 

  • Suppose you have a balance scale and 12 identical looking gold coins.
  • You know that exactly one of the coins is counterfeit and is either heavier or lighter than the other coins.
  • In only 3 weighings, how can you determine which coin is counterfeit and whether it is lighter or heavier?

The children are allowed to share the chocolate coins when they have solved the puzzle.  Can you help them? 

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University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons