Stage: 3 Challenge Level: 
What can you combine with 9?
What can you combine with 20?
What can you combine with 31?
What is special about these numbers?
What is special about their partners?
Creating expressions/formulae. Divisibility. Workshop. Generalising. Modulus arithmetic. Combinations. smartphone. Mathematical reasoning & proof. Properties of numbers. Factors and multiples.
Published January 2009.
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