Maths Olympiad Prep

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Number theory Difficulty 5.5 AIME, harder Prove it Belarus

Let p2p \ge 2 be a prime number. Alice and Bob play the following game: they, in turn, select an index ii in the set {0,1,2,,p1}\{0, 1, 2, \dots, p-1\} that was not selected before by either of the two players and then chooses a digit aia_i. Alice starts. The game ends after all the indices have been selected. The goal of Alice is to make the number
M=a0+10a1+102a2++10p1ap1 M = a_0 + 10 \cdot a_1 + 10^2 \cdot a_2 + \dots + 10^{p-1} a_{p-1}
divisible by pp, and the goal of Bob is to prevent this.
Prove that Alice has the winning strategy.

Solution

2. See IMO-2017 Shortlist, Problem N2.

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