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56th Ukrainian National Mathematical Olympiad, Third Round

Ukraine number theory

Problem

How many three-digit numbers are there with non-zero digits which have the following property: after any permutation of its digits one obtains three-digit number which is divisible by ?
Solution
Obviously, all digits of such number are even because only even digit can be the last one. Also we cannot use or because if a number is divisible by then the last two digits of it are , , , or , , , . So such number consists of the digits and . So:

Three digits (or ) – two numbers.

Two and (or vice versa) – six numbers.

Overall – numbers.
Final answer
8

Techniques

Divisibility / FactorizationCombinatorics