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Mathematica competitions in Croatia

Croatia number theory

Problem

Prove that every positive integer has a positive multiple , such that one can delete one of the non-zero digits of to obtain another multiple of .
Solution
We will find a number in the form where and are digits. Deleting the digit gives the number . Since , to satisfy the conditions of the problem it is sufficient that divides and .

The digit can be chosen so that divides and we set . Then . Let be such that . Then there are integers and such that . If we set , then , obviously a number divisible by .

Techniques

Divisibility / FactorizationModular ArithmeticIntegers