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Steven has 9 gold coins that are identical in appearance. However, one coin is counterfeit an weighs slightly less that the others. Using a balance scale, how can he find the counterfeit coin in just two weighings?

  • Math -

    Split the coins into three groups, A, B and C consisting of 3 coins in each group.

    Weigh: A--B
    1. If A=B, the the counterfeit is in group C
    Weigh any two coins in group C and figure out the rest.

    2. If A≠B, then the counterfeit is in the lighter group.
    Weigh any two coins in the lighter group and figure out the rest.

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