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'Constant Counting' printed from http://nrich.maths.org/
Why do this problem?
is good for helping pupils learn about factors and multiples while becoming more familiar with a calculator.
Handing out calculators one between two pupils will encourage them to communicate while exploring.
When you bring the children together at the end of the lesson, you may want to emphasise some appropriate vocabulary, depending on their age and experience.
(There is some variation in the constant function in different calculators so you may need to experiment a little to find out how to work it. For example, with some older calculators the key sequence is $1 + + = =$.)
What answers have you got?
How many $2$s did you use?
What number could you try next?
Give the pupils a free reign on what calculator buttons to press and challenge them to work out what those they are unfamiliar with do.
It would be good to use this opportunity to listen to the pupils' thinking about what they're doing. You may want to scribe for some children so that they are 'freed up'.