With just 2 weighings out of 9 coins, you can find the fake one
Exactly one of the coins is fake, and it is heavier than the real ones. You can compare their weights only with an unmarked balance scale. What is the minimum number of weighings needed to find the fake coin for certain? Place the coins on the two sides of the scale below and find it yourself.
Key strategy: Divide the coins into three equal groups and put two groups on the scale. If the scale balances, the fake coin is in the group you left off. If one side is heavier, the fake coin is in that heavier group. Because each weighing reduces the candidates to one-third, you can always find the fake coin among N coins in ⌈log₃N⌉ weighings.
Click coins to place them on either side of the scale (click again to remove them)
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💡 Double-click a coin to guess, "This is the fake one!"