http://www.crosswordtools.com/numbers-game/
http://www.dilan4.com/maths/countdown.htm
http://www.danieltebbutt.com/countdown.html
T-SQL
http://developer42.wordpress.com/2013/07/28/countdown-numbers-round-solver-in-t-sql/
http://incoherency.co.uk/countdown/
http://thesamovar.net/countdownnumbers