Stage: 4 Challenge Level: 
This proved quite a challenge! It was Helen (Maidstone Girls' Grammar) and Joel (of ACS in Singapore) who correctly found that only 3 weighings are needed.
Here is one way to solve the problem. We've labelled the coins ABCDEFGHIJKL.
At West Flegg Middle School Polly and Rebecca found a method for 4 weighings, as did Michael (Cranmore Primary School, in Perth, West Australia).
Stage 5 - Reviewed 2012. Generalising. Maximise/minimise/optimise. Manipulating algebraic expressions/formulae. Patterned numbers. Number theory. Mathematical reasoning & proof. Number - generally. Dynamical systems. Inequality/inequalities.
Published April 1998.
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