Weighing 9 Balls

There are 9 balls that are identical. Only one ball is heavier than the rest. You also have a two-armed scale with which to measure the weight of the balls. How can you find the heavier ball by using the scale only twice?

Answer

Split the balls into 3 groups with 3 balls each. Pick 2 groups and use scale to determine which group contains the heavy ball.

Once the group is determined, pick 2 balls and use the scale. If they are equal, the other ball is the heavy one.

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