Stage: 2 and 3 Challenge Level: 
Which pairs of cogs have you found that work?
Which pairs didn't work?
Do you get a sense of why?
What do you notice about the numbers of teeth in each case?
Which pairs didn't work?
Do you get a sense of why?
What do you notice about the numbers of teeth in each case?
Mathematical reasoning & proof. Factors and multiples. Place value. Common factors. Group worthy. Working systematically. Workshop. Properties of numbers. Divisibility. Multiplication & division.
Published May 2010.
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