Three Light Fakes
Eight coins look identical. Exactly three are counterfeit; all counterfeits weigh the same as each other and are lighter than every genuine coin, and all genuine coins weigh the same. You have a two-pan balance scale with no weights, and each weighing must put the same number of coins on each pan. What is the minimum number of weighings that always suffices to determine which three coins are counterfeit?
Related problems & prerequisites
Worth solving first
- Name the Odd Coin 8.0/10
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Source: Multiple-counterfeit variant of the balance-scale puzzle family; original design for AxiomIQ, no traceable source.