Idea 16of 23Builds onSubtraction undoes addition
Count up to subtract
Is there a quick way to work out 10 000 − 9 850?
Today you'll discover
- why counting up works
- how to pick friendly jumps
- how to add up the jumps
Think first10 000 − 9 850 on an open number line
What is 10 000 − 9 850?
Decide first, then press Next. Jump to a friendly number first.
An open number line from 9 850 to 10 000. A first curved jump of +50 lands on 9 900; a second of +100 lands on 10 000. The working reads 50 + 100 = 150. The line is not to scale. The last step asks for 8 003 − 7 996.
See the whole ideathe big idea and the key word, in one place
The big idea
To subtract numbers that are close, count up from the smaller one and add the jumps.
For exampleA walk is 10 000 m and you have gone 9 850 m. Count up: 50 m to 9 900, then 100 m more. 150 m to go.
Key words
- difference
- the answer to a subtraction: how far apart two numbers are
- open number line
- a number line with only the numbers you need, drawn without a scale
Ready to use it?
- Subtract by counting up on an open number line.