olymmath

ISL 2017 N2

    Let p⩾2p \geqslant 2 be a prime number. Eduardo and Fernando play the following game making moves alternately: in each move, the current player chooses an index ii in the set {0,1,…,p−1}\{0,1, \ldots, p-1\} that was not chosen before by either of the two players and then chooses an element aia_{i} of the set {0,1,2,3,4,5,6,7,8,9}\{0,1,2,3,4,5,6,7,8,9\}. Eduardo has the first move. The game ends after all the indices i∈{0,1,…,p−1}i \in\{0,1, \ldots, p-1\} have been chosen. Then the following number is computed: M=a0+10⋅a1+⋯+10p−1⋅ap−1=∑j=0p−1aj⋅10jM=a_{0}+10 \cdot a_{1}+\cdots+10^{p-1} \cdot a_{p-1}=\sum_{j=0}^{p-1} a_{j} \cdot 10^{j} The goal of Eduardo is to make the number MM divisible by pp, and the goal of Fernando is to prevent this. Prove that Eduardo has a winning strategy. (Morocco)

    Dataset text NuminaMath olympiads. Quoted with source.

    Official text: IMO 2017 Shortlist booklet, page 12.

    Solutions

    Official solution Solution 1 by the IMO Problem Selection Committee

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