9 People in 12 chairs

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If 9 people are seated in a row of 12 chairs, then some consecutive set of 3 chairs are filled with people.

Solution

If 9 people are seated in a row of 12 chairs, then some consecutive set of 3 chairs are filled with people.

The 3 empty chairs partition the filled chairs into 4 groups of consecutive filled chairs. Since 9/4>2, some group has at least 3 chairs filled with people.

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