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Ukrajina 2008

Ukraine 2008 number theory

Problem

We know that at some natural the number written in decimal notation ends with digit 3. Find what digits are in the hundred's and ten's place of the number.
Solution
It's clear that the number does not influence the answer therefore the sought digits are the same for numbers and .

As the number equals (being the cube of the natural number) and ends in 0, this number should end in 000. Thus where are some natural numbers. Therefore the last three digits of the number are 073.
Final answer
hundreds digit = 9; tens digit = 7

Techniques

Modular ArithmeticPolynomials