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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? 

Please email your answers to deputy-director[at][dot]uk and remember to include your name (or the name of the group that you solved the puzzle with) so that we can publish it on our website if you are one of the first few to successfully solve the problem. Please be aware that until Monday 19 August replies to each submission may be slightly delayed due to INI staff annual leave, but do be reassured that each message is read and greatly appreciated.  

Answers to all puzzles will be published at the end of the month. 

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