Baltic Way 2011 · Shortlist problem
Number Theory
All ten-digit numbers composed of digits and are divided by (with the remainder). How many different reminders are obtained by these calculations?
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Review
Topics
Modular arithmetic
Solutions
Solution
Answer: . 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 , , and the quotient is odd. Therefore the difference is divisible by and not by , so it is not equal to modulo .