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MathNet 2008-part-01 — 165 problems (EN–VI) Question 31

Math Olympiad Vault · MathNet 2008-part-01 — 165 problems (EN–VI)

Question 31

Twenty integers are written around a circle.
For each of them, the sum of the $10$ numbers that follow it in the clockwise direction is computed. When this is done, each of the $20$ numbers is replaced by its corresponding sum.
Prove that after repeating this process several times, each of the $20$ numbers around the circle will be even.