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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? 

Entry to this puzzle is now closed.  Congratulations to the following for some great answers: 

  • Ali Raad
  • Simon I Hansen
  • Evgeny Mirkes
  • Junyang Wang
  • Emily Lies and Jan Ricken
  • Sarisha Srikanthapalan
  • Niall Moran and Jen McSharry
  • Fin and Rebecca Scott
  • Krishi
  • Kanha Seksaria
  • Soham Sud
  • Lindsay Woodhead
  • Alihan Huyuk Farrest
  • Cheng Yuam Li
  • Benjamin Dadoun
  • Patrick Luo
  • Ben Katz


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