You have two coins, A and B. One is genuine, the other is fake. You also have onegenuine coin G. You don't know if the fake coin is heavier or lighter than the genuine coin. Use a balance scale one time and figure out which coin is fake.

• What we did:
  • We put A on the scale's left side
  • We put G on the right side
  • The scale balanced
• What does this mean?
  • A and G are exactly the same weight.
  • Obviously, the fake isn't on the scale.
  • Coin B must be the counterfeit.