Stage: 2 Challenge Level: 
There are lots of answers to this, depending on what questions you choose to ask.
Have a go yourself, and if you discover anything interesting then let us know ! Please don't worry that your solution is not "complete" - we'd like to hear about anything you have tried. Teachers - you might like to send in a summary of your children's work.
Pinboard/geoboard. Combinations. Visualising. Addition & subtraction. Mixed triangles. Working systematically. Interactivities. Practical Activity. To be developed. Investigations.
Published March 1998.
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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