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Baltic Way 2011 · Shortlist problem

Number Theory

All ten-digit numbers composed of digits 11 and 22 are divided by 10241024 (with the remainder). How many different reminders are obtained by these calculations?

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Modular arithmetic

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Solution

Answer: 10241024. All the reminders are pairwise distinct, because it is not difficult to see that the difference between any two numbers have an odd digit and several zeroes at the end of its decimal representation. Therefore it is divisible by 10k10^k, 0≤k≤90 \le k \le 9, and the quotient is odd. Therefore the difference is divisible by 2k2^k and not by 2k+12^{k+1}, so it is not equal to 00 modulo 10241024.