Detailed 3-step strategy: weigh 4 vs 4 to find suspect group; weigh 3 suspects vs 3 known genuine coins to narrow suspects; weigh final coins to identify the lighter counterfeit among 12 coins in 3 weighings.
12 coin problem
counterfeit coin identification
balance scale weighing strategy
three weighings solution
Identify light counterfeit among 12 coins in 3 weighings