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There are five cards on the table with positive integers written on both sides. The numbers, in order, 1, 2, 6, 7, and 15 are written on their visible sides. I claim that we can flip over some cards so that both sides show arithmetic sequences, without reordering the cards.

What is the smallest number of cards we must turn over to determine whether or not I am is correct?

 Sep 22, 2019

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