olymmath

ISL 2017 N4

  • Answer

Call a rational number short if it has finitely many digits in its decimal expansion. For a positive integer mm, we say that a positive integer tt is mm-tastic if there exists a number c∈{1,2,3,…,2017}c \in\{1,2,3, \ldots, 2017\} such that 10t−1c⋅m\frac{10^{t}-1}{c \cdot m} is short, and such that 10k−1c⋅m\frac{10^{k}-1}{c \cdot m} is not short for any 1⩽k<t1 \leqslant k<t. Let S(m)S(m) be the set of mm-tastic numbers. Consider S(m)S(m) for m=1,2,…m=1,2, \ldots. What is the maximum number of elements in S(m)S(m) ? (Turkey)

Dataset text NuminaMath olympiads. Quoted with source.

Official text: IMO 2017 Shortlist booklet, page 12.

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807

Solutions

Official solution Solution 1 by the IMO Problem Selection Committee

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