The world's 23 toughest math questions [http://www.networkworld.com/community/blog/worlds-23-toughest-math-questions] - 2013-10-05 21:17:46 - public:time challenge, math, problems, puzzles - 4 | id:1968 -
Four Fours [http://www.wheels.org/math/44s.html] - 2010-04-21 00:22:30 - public:time fun, math, mathematics, puzzles - 4 | id:3317 -
Puzzles [http://www.puzzleworld.org/] - 2007-11-13 19:38:03 - public:time games, puzzles, wood - 3 | id:4298 - http://www.puzzleworld.org/DesignCompetition/2006/entries.htm has more specific detials