Recently my close friend and co-author Bob Albrecht shared with me examples of a category of math problem used in some elementary school math classes. We are currently co-authoring a book on the use of games to enhance math education for K-8 students. (Note added 12/2/2012: See Moursund & Albrecht in Reference at the end of this entry for the title and a link to download the book.)
Add to 15: List as many ways as you can to use the numbers 1 through 9 to add up to 15, without repeating any of the numbers in a single equation. If you can, try to list all possible ways.