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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.

Solution with three weighings

At West Flegg Middle School Polly and Rebecca found a method for 4 weighings, as did Michael (Cranmore Primary School, in Perth, West Australia).