Stage: 3 Challenge Level: 
Since this problem can be tackled in a variety of ways it can lead to useful discussions on the merits of different strategies.
If students disagree about the correct answer they can be challenged to develop convincing arguments that justify their conclusions.
Workshop. Generalising. Venn diagrams. Multiplication & division. Common factors. Divisibility. Working systematically. Factors and multiples. Prime factors. Properties of numbers.
Published November 1999.
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