You are given 18 silver coins but you find out that one of the coins(you kon't know which ) is fake and weighs less than any of the others.
How can you be able to detect the false coin using a normal balance scale only three times?
1. The eighteen coins are divided into three groups of 6. Two of these groups are weighed against each other. This enables you to determine which of the three groups contains the false coin.
2. The group of 6 coins that contains the false coin is divided into three pairs of coins and two of these pairs are weighed against one another. This allows you to determine which pair contains the false coin.