Not easy
Source: Somewhere
June 14, 2004
vectorcombinatorics proposedcombinatorics
Problem Statement
Given a table with 1*1 squares ( rows and n column). In any square we put a number in {1, -1} such that no two rows are the same. Then we change numbers in some squares by 0. Prove that in new table we can choose some rows such that sum of all numbers in these rows equal to 0.