One of the coins A, B and C, is a fake, heavier than the other two. We only need to use a balance scale once to determine which is the fake.

• What we did:
  • We put A on the scale's left side
  • We put B on the right side
  • The left side of the scale went down
• What does this mean?
  • We know that the fake is the heavier coin
  • The fake must be on the side that went down
  • Coin A must be the counterfeit.