Examples of what pupils should know and be able to do
Work out mentally that:
4005 – 1997 = 2008 because it is
4005 – 2000 + 3 = 2005 + 3 = 2008
Work out mentally by counting up from the smaller to the larger number:
8000 – 2785 is 5 + 10 + 200 + 5000 = 5215
Probing questions
Which of these subtractions can you do without writing anything down?
Why is it possible to work this out mentally? What clues did you look for?
How did you find the difference? Talk me through your method.
If 2003 is the answer to a similar question, what could the question be?
What if pupils find this a barrier?
Pupils may need to go back to calculating easier differences using a number line.
It is important for pupils to be able to visualise an appropriate part of a number line so they can recognise when numbers are close to a multiple of 10, 100 or 1000. Ask pupils to imagine a number line and to locate specific numbers on it. Progressively ask pupils to identify the numbers they need on their number line to be able to find the difference.
Counting on and back ITP - Zip file (456Kb)
Number Line ITP - Zip file (459Kb) will form a useful visual image to support pupils' mental calculations
Refer to Teaching mental calculation strategies to L5 - Acrobat pdf document (554Kb) pages 31-44.
Refer to Teaching mental calculation strategies to L5 - Acrobat pdf document (554Kb) pages 15-30.
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