You have given a two-arm balance scale and 9 identical looking balls. One of the ball is of different weight (heavier) than rest of the 8 balls. Find out the heavier weight ball by using two arm balance scale. You could use a pair of balance scales only twice.
Solution
Divide the 9 balls into three sets, each containing 3 balls each. Let’s name them for easy understanding. A {3 balls}, B {3 balls}, C {3 balls}. Now weigh them against each other to find out the set containing different weight ball. Weigh A vs. B, either the scale will balance or the scale will not balance and one side will be heavier. If the scale is balanced, then the different weight ball must be in C else the heavier group should then be obvious.
Till now we have identified that different weight ball group. Now, choose 2 balls from this group and compare their weights, and using the same logic as before, the heavier ball will be obvious.