What is Mental Subtraction?

Tiffany Maths dress

Tiffany likes maths and loves to learn how to calculate. She likes to wear her numbers dress when she is learning maths.

Tiffany has been learning about mental subtraction.

Did you know?

Mental subtraction means taking one number away from another in your head (without using pen and paper).

Being able to subtract numbers in your head is a good skill to learn and will save time with calculations.

The more you practice mental subtraction the easier it will become.

 

Using number bonds to help with mental subtraction

Learning as many number bonds as possible will definitely help with mental maths.

If you know your number bonds to ten then you will know immediately that

10 – 2 = 8

If you know your number bonds to twenty then you will know immediately that

20 – 15 = 5

If you know your number bonds to one hundred then you will know immediately that

100 – 70 = 30

You can learn number bonds to any number – 10, 20, 30, 40, 100, 1,000 etc

 

Mental subtraction by counting backwards

If you can count backwards by 1s, 2s, 5s, 10s  it will help with subtracting

counting backwards in 1s – 

For 18 – 5, count backwards from 18 by 5 single numbers

18 – 5 = (17,16,15,14) 13

counting backwards in 2’s

For 30 – 8, count backwards from 30 by 2 four times

30 – 8 = (28,26,24) 22

counting backwards in 5’s

For 50 – 15, count backwards from 50 by 5 three times

50 – 15 = (45,40) 35

counting backwards in 10’s

For 100 – 40, count backwards from 100 by 10 four times

100 – 40 = (90,80,70) 60

 

Using mental subtraction to take away large numbers

Subtracting larger numbers in your head is much easier if you break the numbers up. This is called chunking or partitioning.

Here is an example of how this works:

45 – 36 = ?

Using partitioning, 36 is the same as 30 + 6 so we can subtract 30 and then 6

45 – 30 = 15 and 15 – 6 = 9 

so

45 – 36 = 9

 

This method works with bigger numbers

226 – 133 = ?

Using partitioning, 133 is the same as 100 + 30 + 3, so we can subtract 100, then 30, then 3

226 – 100 = 126,  126 – 30 = 96,  96 – 3 = 93

so

226 – 133 = 93

 

Tiffany has enjoyed learning about mental subtraction and now knows how to take away numbers in her head using number bonds and partitioning. She summed up her learning with the following keywords:

calculation, counting, mental, number bonds, partitioning, subtraction, taking away