User:Erall

Seperate the coins into 3 piles: A, B, and C Weigh A against B Weigh B against C

One of 6 cases: A > B = C (A contains a heavy coin) A < B = C (A contains a light coin) A = B < C (C contains a heavy coin) A = B > C (C contains a light coin) A < B > C (B contains a heavy coin) A > B < C (B contains a light coin)

In each case, you know which pile contains the odd coin and whether the odd coin is light or heavy.

Weigh two coins from the odd group against each other. If they balance, the third coin is the odd one. Otherwise, the lighter or heavier one is odd, and you know which from the six cases.