Copyright © University of Cambridge. All rights reserved.

'Constant Counting' printed from http://nrich.maths.org/

Show menu

Constant Counting


You can make a calculator count for you by pressing the keys

$1 + = = $

$1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12$

To make it count by twos, press $2 + = = $

$2, 4, 6, 8, 10, 12$

You can make it count by any number you choose.

Now the question:

You can count by ones to reach $24$.
You can count by twos to reach $24$.
What else can you count by to reach $24$?

 

Why do this problem?

This activity is good for helping pupils learn about factors and multiples while becoming more familiar with a calculator.

Possible approach

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 $1 + + = =$.)

Key questions

What answers have you got?
How many $2$s did you use?
What number could you try next?

Possible extension

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.

Possible support

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'.