Interview Brain Teasers Hard 2 min

You have 9 identical-looking balls and one is heavier. What is the minimum number of balance-scale weighings needed to find it?

Hint

Divide the balls into three equal groups.

Answer

2 weighings.

Explanation

Weigh 3 against 3. If they balance, the heavy ball is in the remaining three. If they do not, it is in the heavier group. A second weighing among those three identifies it.

Step-by-Step Solution

First weighing: 3 vs 3.
Select the group containing the heavier ball.
Second weighing: 1 vs 1 from that group.
If they balance, the third ball is heavier.
Otherwise the heavier side identifies it.

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