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Junior Regional Olympiad - FBH 2015 Grade 8 Problem 5

Source:

October 1, 2018
Setsdisjoint

Problem Statement

Prove that for every parititon of set X={1,2,...,9}X=\{1,2,...,9\} on two disjoint sets at least one of them contains three elements such that sum of some two of them is equal to third