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boxes and balls in a circle

Source: Nigerian Mathematics olympiad 2021 round 2 problem 2

February 14, 2021
combinatorics

Problem Statement

NN boxes are arranged in a circle and are numbered 1,2,3,.....N1,2,3,.....N In a clockwise direction. A ball is assigned a number from1,2,3,....N{1,2,3,....N} and is placed in one of the boxes.A round consist of the following; if the current number on the ball is nn, the ball is moved nn boxes in the clockwise direction and the number on the ball is changed to n+1n+1 if n<Nn<N and to 11 if n=Nn=N. Is it possible to choose NN, the initial number on the ball, and the first position of the ball in such a way that the ball gets back to the same box with the same number on it for the first time after exactly 20202020 rounds