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Sunday Afternoon Maths XLIV

 Posted on 2015-07-26 

Rotating round table

At a large dinner, 24 people are to sit evenly spaced around a round table. Place cards are laid to show where everyone should sit. Unfortunately nobody notices the name cards and the guests sit down with nobody in the correct seat.
Show that it is possible to rotate the table so that at least two people will be in the correct seats.

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Tags: numbers

Balanced sets

A set of points in the plane is called 'balanced' if for any two points \(A\) and \(B\) in the set, there is another point \(C\) in the set such that \(AC=BC\) (here \(AC\) is the distance between \(A\) and \(C\)).
For all \(n\geq3\), find a balanced set of \(n\) points.

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121

Find a number base other than 10 in which 121 is a perfect square.

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Tags: numbers, bases
If you enjoyed these puzzles, check out Sunday Afternoon Maths LXVII,
puzzles about triangle numbers, or a random puzzle.

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