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Choosing Digits: Making a Sum Correct

In this worksheet, students choose the missing digits needed to make the given statements correct.

'Choosing Digits: Making a Sum Correct' worksheet

Key stage:  KS 2

Curriculum topic:   Number: Addition and Subtraction

Curriculum subtopic:   Solve Add/Subtract Problems to 1000

Difficulty level:  

Worksheet Overview

Are you ready to go on a number hunt?  In the sums below some of the numbers are missing and instead are replaced by a  


Little girl looking through a magnifying glass


How do we solve this?  Let's have a look at this first sum...

We have to find a digit (another word for number) from 0-9 to make this sum correct.  


3♦  -  5  = 25


How do we solve this question? First of all, we need to look at the operation.  (That just means whether the sum uses an addition, subtraction, multiplication or division sign)  

In this case, we can see the operation used is a subtraction

Here's the handy tip for working out operations using subtraction.


1) We re-arrange the sum by swapping 25 with 3♦ in the order to get:

 25 -  5  =  3♦


2) Then we change the sign so that it uses the opposite (or inverse) sign.  The opposite of subtracting is adding, so we write:

25 + 5  =  3♦


3) Now we solve the sum:

25 + 5  =  30


4) The last thing we do is put our number back in the original sum to check our answer:

30  -  5  = 25



Now we move on to try to solve a sum where two numbers are missing.  Looks much harder?  Don't panic we can apply the same clever tip!


Let's look at the sum below:

8  -  4  = 23


First of all, let's have a quick think about what each number represents?

8: In this number the 8 means 8 tens, and the ♦ stands for the units.

4: In this number the  stands for the tens, and the stands for the 4 units.

23:  In this number the 2 stands for 2 tens, and the 3 stands for three units.


We need to start with one missing number first so we'll start with the missing red number.  Ignore the green missing number for now!

The red missing number represents the units.  So, for now, we're going to gather all of the units in the sum above (the second number) together.

- 4 = 3


Do you remember what we did last time with a sum where the operation was a subtraction?  We rearranged it first by swapping the first number and the number after the equals over:

3 - 4 = 


Then we replaced the subtraction sign with an addition sign (as this was the opposite):

3 + 4 = 


Then we solved the sum:

3 + 4 = 7


Then we checked it back in the original sum to check it made sense:

- 4 = 3


We can put the 7 into our sum below as we know this is our missing unit...but remember...we have two missing numbers this time!  

So now we have:

87  4  = 23


To find out our second missing number (the missing tens digit) we can do exactly the same process:

Gather the tens together:

8 -  = 2


Rearrange the sum by swapping the first number and the number after the equals sign to get:

-  = 8


Change the subtraction sign to an addition sign (as these are opposite) to give:

+  = 8


Solve the sum:

+ 6 = 8

Check this number back in the original sum:

- 6 = 2


When we put the number back in our original sum with two missing numbers we now have the statement 87  -  64  = 23

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