Stage: 4 Challenge Level: 
The 'trick' is finding a systematic way to record all possibilities.
Algorithms. Codes and cryptography. Mathematical reasoning & proof. Mathematical induction. Networks/Graph Theory. Combinatorics. Fibonacci sequence. epsilons. Dynamical systems. Patterned numbers.
Published September 2000,February 2009.
Copyright © 2007. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project, which also includes the Plus and Motivate sites.
email: nrich@damtp.cam.ac.uk