In 80 coins one coin is counterfeit. What is minimum number of weighings to find out counterfeit coin?
PS: The counterfeit coin can be heavy or lighter.
2026-05-15 04:10:42.1778818242
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A coin-weighing puzzle with 80 coins
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Hint: Consider weighing one third of the coins on each side. What will this tell you?
In fact, it can be shown that the process, the definition of which you will be led to by this hint, is optimal. Try to think of why this is the case, and to classify how many weighings you need with $n$ coins.
Hint: Consider weighing half of the coins on each side. What will this tell you ?