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IMO 2024 — Complete Shortlist Question 14

IMO Shortlist · IMO 2024 — Complete Shortlist

Question 14

Let $n$ and $T$ be positive integers. James has $4n$ marbles with weights 1, 2, ..., $4n$. He places them on a balance scale, so that both sides have equal weight. Andrew may move a marble from one side of the scale to the other, so that the absolute difference in weights of the two sides remains at most $T$.

Find, in terms of n, the minimum positive integer T such that Andrew may make a sequence of moves such that each marble ends up on the opposite side of the scale, regardless of how James initially placed the marbles.