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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 4. Find what digit is in the ten's place of the number.
Solution
It's clear that the number does not influence the answer, which implies that the sought digits will be the same for numbers and . As number equals (being the square of the natural number) and ends in , this number should end in . Thus where are some natural numbers. Therefore the last two digits of the number are .
Final answer
8

Techniques

OtherPolynomial operations