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Two light fakes among four coins

weighingbalance-scalefake-coincase-analysis

You have four coins, A, B, C and D. Exactly two of them are counterfeit. Both fakes are lighter than a genuine coin, and both fakes weigh the same as each other. You have a two-pan balance.

What is the smallest number of weighings that guarantees you identify both fake coins? Describe the procedure.

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There are six ways to choose which two of the four coins are fake. Three outcomes per weighing means one weighing cannot separate six cases. Think about what "balanced" tells you when there are two fakes.

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