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Greece Junior National Olympiad 1977-78 problem 3

Source:

August 10, 2015
number theory

Problem Statement

Let kk be a prime, such as k2,5k\neq 2, 5, prove that between the first kk terms of the sequens 1,11,111,1111,....,1111....11, 11, 111, 1111,....,1111....1, where the last term have kk ones, is divisible by kk.