IMO Shortlist 2017 problem N4


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3. listopada 2019.
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Call a rational number short if it has finitely many digits in its decimal expansion. For a positive integer m, we say that a positive integer t is m-tastic if there exists a number c\in \{1,2,3,\ldots ,2017\} such that \dfrac{10^t-1}{c\cdot m} is short, and such that \dfrac{10^k-1}{c\cdot m} is not short for any 1\le k<t. Let S(m) be the the set of m-tastic numbers. Consider S(m) for m=1,2,\ldots{}. What is the maximum number of elements in S(m)?

Izvor: https://www.imo-official.org/problems/IMO2017SL.pdf