Similar to "chestnuts" for riddles, sometimes math puzzles are based on things that have already been done. For example, a recent magic-square question has an answer that is very easily searchable, with a solution that goes back a bit over a century.
Do I simply copy/paste the solution and explain it a bit (along with attribution, of course)? Should I wait a while and let people puzzle it over instead? By "easily searchable" I mean it's within the top few hits on Google for somewhat obvious search terms, so I don't expect it will be long before someone finds and/or answers it.