Mental Subtraction

Tiffany with rucksack

 

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

Being able to take away numbers in your head is a good skill to learn and will save time

There are a number of different tips and tricks to help with mental subtraction

 

 

Know Your Number Bonds

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 - 5 = 15

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

 

Break Numbers Up (Chunking)

If you are asked to take 36 away from 45 in your head it is much easier to do 45 - 30 - 6

45 - 30 = 15 - 6 = 9

Similarly, 226 - 133 is much easier as 226 - 100 - 30 - 3

226 - 100 = 126 - 30 = 96 - 3 = 93

 

Learn to Count Backwards

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

counting backwards in 1's

18 - 5 = (17,16,15,14) 13

counting backwards in 2's

30 - 8 = (28,26,24) 22

counting backwards in 5's

50 - 15 = (45,40) 35

counting backwards in 10's

100 - 40 = (90,80,70) 60

 

Harvard Reference for this page:

Heather Y Wheeler. (2015). Mental Subtraction. Available: http://www.tiffanymacbeth.com/Maths/mental_subtraction.htm. Last accessed


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Page last updated 02/04/2016

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