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Print56th 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.
Three digits (or ) – two numbers.
Two and (or vice versa) – six numbers.
Overall – numbers.
Final answer
8
Techniques
Divisibility / FactorizationCombinatorics