olymmath

ISL 2009 C7

    A grasshopper jumps along the real axis. He starts at point 0 and makes 2009 jumps to the right with lengths 1,2,…,20091,2, \ldots, 2009 in an arbitrary order. Let MM be a set of 2008 positive integers less than 1005 - 2009. Prove that the grasshopper can arrange his jumps in such a way that he never lands on a point from MM. Variant 2. Let nn be a nonnegative integer. A grasshopper jumps along the real axis. He starts at point 0 and makes n+1n+1 jumps to the right with pairwise different positive integral lengths a1,a2,…,an+1a_{1}, a_{2}, \ldots, a_{n+1} in an arbitrary order. Let MM be a set of nn positive integers in the interval (0,s)(0, s), where s=a1+a2+⋯+an+1s=a_{1}+a_{2}+\cdots+a_{n+1}. Prove that the grasshopper can arrange his jumps in such a way that he never lands on a point from MM.

    Official text Transcription from NuminaMath olympiads, checked word for word against the official paper. Quoted with source.

    Official text: IMO 2009 Shortlist booklet, page 9.

    Solutions

    Official solution Solution 1 by the IMO Problem Selection Committee

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