Answer 11: Only 2 weighings ⚖️
Split the coins into three groups of 3. Weigh group A against group B. If they balance, the fake is in group C. If not, the lighter side has the fake. Second weighing: take the group containing the fake, weigh coin 1 vs coin 2. If they balance, coin 3 is fake. If not, the lighter one is fake.
