Alex Hedley - Countdown Idea

http://www.crosswordtools.com/numbers-game/

http://djce.org.uk/countdown

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://stackoverflow.com/questions/15293232/how-to-design-an-algorithm-to-calculate-countdown-style-maths-number-puzzle

http://stackoverflow.com/questions/16510634/24-game-countdown-number-game-solver-but-without-parentheses-in-the-answer

http://incoherency.co.uk/countdown/ https://github.com/jes/cntdn/

http://thesamovar.net/countdownnumbers

https://github.com/wharris/countdown