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 $$6-4$$ is just counting backwards from six by four. If you know how to count backwards, you know how to subtract.

Example 1

Calculate

$$8-3$$​​

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

So, 

$$8-3=\underline{5}$$

​​​​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 

$$97-35$$​​

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

​​

So,

$$97-35=\underline{62}$$​​

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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

$$215-76$$​​

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 $$209$$:

Notice that when subtracting the tens, you have changed the hundreds digit. This is because you subtracted to go below $$0$$ in the tens column. So,

$$215-76=\underline{139}$$​