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Subtracting numbers using mental maths

Subtracting numbers using mental maths

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Tutor: Kat

Summary


Subtracting numbers using mental maths

​​In a nutshell

By keeping track of your units, tens, hundreds, or other place values with many digits, you can subtract numbers using mental maths.



Subtracting a small number

Subtracting is just counting down. So 6−46-46−4 is just counting backwards from six by four. If you know how to count backwards, you know how to subtract.


Example 1

Calculate

8−38-38−3​​


This is the same as counting backwards from eight by three. 


So, 

8−3=5‾8-3=\underline{5}8−3=5​



​​​​Subtracting a larger number

For larger numbers, counting in jumps of one will take too long. Instead, you break down the number you are subtracting into its units, tens, hundreds, and other place values, and count those jumps individually.


Example 2

Calculate 

97−3597-3597−35​​


Start by subtracting the units. There are five units to subtract. Next, subtract the three tens.

​​

So,

97−35=62‾97-35=\underline{62}97−35=62​​​

​


Changing other place values

When subtracting, you may often be just subtracting units but have to change the tens digit too. This may be true of any place value: you could be subtracting thousands, but have to change the tens of thousands value.


Example 3

Calculate

215−76215-76215−76​​


Start by subtracting the six units. Notice that the tens unit has dropped by one even though you were subtracting the units. Next, subtract the seven tens from 209209209:


Notice that when subtracting the tens, you have changed the hundreds digit. This is because you subtracted to go below 000 in the tens column. So,

215−76=139‾215-76=\underline{139}215−76=139​​


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Learn with Basics

Learn the basics with theory units and practise what you learned with exercise sets!
Length:
Numbers to 1000

Unit 1

Numbers to 1000

Adding numbers using mental maths

Unit 2

Adding numbers using mental maths

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Optional

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Subtracting numbers using mental maths

Unit 3

Subtracting numbers using mental maths

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FAQs - Frequently Asked Questions

What is subtraction?

Subtracting is just counting down. It is reducing the value of your number. So 6−4 is just counting backwards from six by four. If you know how to count backwards, you know how to subtract.

How can you subtract mentally?

Imagine a number line and count backwards by the number you are subtracting.

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