IMO 2020 #5

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FuadAlAlam
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IMO 2020 #5

Unread post by FuadAlAlam » Fri Dec 04, 2020 12:48 pm

A deck of $n > 1$ cards is given. A positive integer is written on each card. The deck has the property that the arithmetic mean of the numbers on each pair of cards is also the geometric mean of the numbers on some collection of one or more cards.
For which $n$ does it follow that the numbers on the cards are all equal?

Proposed by Oleg Košik, Estonia

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