Now you have 9 coins. All of them look the same. Exactly 1 is counterfeit, and it is lighter than the true coins.
All true coins weigh the same amount.
You may use a set of balance scales.
Can you isolate the counterfeit coin with only 2 weighings?
Exactly 1 of these 9 coins is counterfeit, and it is lighter. Use at most 2 weighings to isolate it.
The branch preview is most useful here: look for a first weighing that leaves a three-way split as close to 3-3-3 as possible.
Left pan
Drop coins here
Right pan
Drop coins here
Coin tray
Drag coins from here onto either pan
Preview = case split; tap coins to move tray ↔ pans.
9 cases match history.
Pick the fake from the full set of coins.
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