Browse · MathNet
PrintMathematica 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 .
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