MathDB
Putnam 2010 B3

Source:

December 6, 2010
Putnamnumber theorygreatest common divisorpigeonhole principlerelatively primecollege contests

Problem Statement

There are 2010 boxes labeled B1,B2,,B2010,B_1,B_2,\dots,B_{2010}, and 2010n2010n balls have been distributed among them, for some positive integer n.n. You may redistribute the balls by a sequence of moves, each of which consists of choosing an ii and moving exactly ii balls from box BiB_i into any one other box. For which values of nn is it possible to reach the distribution with exactly nn balls in each box, regardless of the initial distribution of balls?