Stage: 2 Challenge Level: 
Try with different numbers below $50$. It may help to make a version going across the page with arrows or you could use this sheet.
Trial and improvement. Interactivities. Generalising. Addition & subtraction. Functions and their inverses. Combinations. Factors and multiples. Investigations. Multiplication & division. Working systematically.
Published July 2003,August 2003.
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