Inverse Operations to Check Calculations | Year 4 Maths Resources

## What you need to know

Things to remember:

• Addition is the opposite of subtraction.
• Multiplication is the opposite of division.

Sometimes it is important to know how to check answers, which we will do here by using inverse operations. This is a fancy way of saying “opposite symbol”.

Check to see if $52+6=58$ by using the inverse operation.

To check this, we use the opposite of addition, which is subtraction. So, instead of adding 6, we subtract 6. But what are we subtracting the 6 from? Our original answer!

$$58-6=52$$

(You could subtract either number, but subtracting 6 is nicer)

These are the numbers we started with, so $52+6=58$ is correct.

Check to see if $7\times4=28$ by using the inverse operation.

To check this, we use the opposite of multiplication, which is division. So, instead of multiplying by 4, we divide by 4. But what are we dividing by 4? Our original answer!

$$28 \div 4 = 7$$

(You could divide by either number, but 4 is nicer)

These are the numbers we started with, so $7\times4=28$ is correct.

We have to be a little careful when doing subtraction and division questions, we have to use the number on the right of the symbol.

Check to see if $15-9=8$ by using the inverse operation.

To check this, we use the opposite of subtraction, which is addition. So, instead of subtracting 9, we add 9. But what are we adding 9 to? Our original answer!

$$8+9=17$$

(We have to add the 9)

Here we have got a different answer, so $15-9=8$ is not correct.

Check to see if $27\div9=3$ by using the inverse operation.

To check this, we use the opposite of division, which is multiplication. So, instead of dividing by 9, we multiply by 9. But what are we multiply by 9? Our original answer!

$$3\times9=27$$

(We have to multiply by the 9)

Here we got our original numbers, so $27\div9=3$ is correct.

## Example Questions

$24-9=15$

$15+9=24$ is correct.

$8\times4=32$

$15+9=24$ is not correct.

$9\times4=36$