Form three (3) main groups – each containing 1 and 3 coin sub-groups. Place one main group on each pan of a two pan balance – the third main group on the table. Observe the condition of the balance. This is the first weighing.
Rotate the 3 coin sub-groups (from pan A to the table, pan B to pan A, and from the table to pan B. Observe the condition of the balance. (second weighing).
If there is a change, it will identify the 3 coin group that contains the odd coin and its relative weight. In this case, clear the area of all other coins. Split this group into single coins, placing two on the pans of the balance. This willl be the third weighing and will identify the odd coin. We already know its relative weight. Problem solved.
If there is no change, one of the single coins (two of which are already on the pans of the balance) is the odd coin. In this case, rotate the single coins. This will be the third weighing an will identify the odd coin and its relative weight. Problem solved.
