Missing Numbers in Subtraction
Basics on the topic Missing Numbers in Subtraction
Finding Missing Numbers in Subtraction
When solving missing number subtraction problems we need to check which number in the equation is missing. Sometimes instead of finding the difference, or answer, we have to find the number that is being subtracted:
Sometimes we have to find the number we started with.
When working on subtraction missing number problems, there are two different ways to solve.
Solving Subtraction Problems with Missing Numbers
In the number sentence above, 5  _ = 3, we know five minus something equals three.
In order to solve this problem, we can rewrite the number sentence by moving the number from after the equals sign to before the equals sign.
Now solve five minus three to get the answer, two. Fill in the missing number (subtraction), 2. No matter what you are working out, always check your answer using the original number sentence.
Next, the first number is missing in the number sentence.
We know that something minus two equals three. In order to find the missing number, we can rewrite the equation as three plus two, to get the answer five. No matter what you are looking for, always check your answer using the original number sentence.
Missing Numbers in Subtraction – Summary
How do you find the missing number in subtraction? The answer depends on the missing numbers for subtraction in the number sentence. Follow these steps to find the missing number in a subtraction problem.
Step #  What to do 

1  Find out which number is missing in the problem. 
2  Rewrite the number sentence by moving the number from after the equal sign to before the equal sign. 
3  No matter what you are solving for, always check your answer using the original number sentence. 
Sometimes instead of finding the difference, or answer, we have to find the number that is being subtracted. No matter what you are looking for, always check your answer using the original number sentence.
Want to practise identifying missing numbers in subtraction? In this video you can find missing numbers in subtraction examples and on this website there is a missing numbers subtraction worksheet along with other missing numbers subtraction KS2 activities, and exercises.
Transcript Missing Numbers in Subtraction
Last night, Imani heard some strange noises, but decided to wait until the morning to see what was going on. Oh no! What happened!? Someone must have helped themselves to some snacks last night! Let's help Imani identify "Missing Numbers in Subtraction" so they can investigate! Subtract means to take away from a group. We solve a number sentence by subtracting to find the answer. Sometimes, instead of finding the difference, or answer, we have to find the number that is being subtracted OR the number we started with. For example, Imani had thirtysix pieces of corn and now they have five. They want to know how many pieces were eaten, so we need to find the missing number in the number sentence. First, look at the number sentence. Thirtysix minus SOMETHING equals five. We can find the DIFFERENCE between 36 and 5, or we can rewrite the number sentence by moving the number from AFTER the equals sign to BEFORE the equals sign. Now solve THIRTYSIX minus FIVE. What is thirtysix minus five? Thirtyone. SOMEONE ate thirtyone pieces of corn. Let's check our answer by putting thirtyone in the ORIGINAL number sentence. Thirtysix minus thirtyone equals five. Now, Imani wants to know how many muffins they started with. They found fortyeight muffin wrappers, and there are twentyone muffins left. We need to find the missing number in the number sentence. SOMETHING minus fortyeight equals twentyone. In order to find the missing number, we can count all of the muffins and muffin wrappers, or we can rewrite the equation as FORTYEIGHT plus TWENTYONE. If we add fortyeight and twentyone, we will find the number of muffins they started with. What is fortyeight plus twentyone? Sixtynine, so they started with sixtynine muffins. Let's check our answer by putting sixtynine in the ORIGINAL number sentence. Is it correct? Sixtynine minus fortyeight is twentyone, so they started with sixtynine muffins. Now that we've collected all of the details about the snacks, Imani will go into Investigation Mode to try and find out who ate them! Before they do that, let's summarise. Remember, when solving a subtraction number sentence, sometimes we may have to find the number we started with like this one HERE. We can solve this by rewriting the number sentence and ADDING the numbers that we have. Also, we may have to find the number that is being subtracted like in THIS number sentence. We can solve this by rewriting the number sentence and SUBTRACTING the numbers that we have. No matter what you are working out, always check your answer using the original number sentence. Let's see what Imani's Investigation Mode found!
"Heh heh, I must have been sleep walking again!"
Missing Numbers in Subtraction exercise

How many bananas were eaten?
HintsRemember, you can switch the number sentence around and subtract 15 from 32 to find the answer.
Use the image to help you. How many banana skins are there?
SolutionSomeone has eaten 17 bananas.
To find the answer, we can switch the number sentence around and solve 32  15, which equals 17.

How many of each item has been eaten?
HintsIn this example, there were 25 buns and Imani found 11 wrappers, so we want to work out the difference between 25 and 11. How could we solve this?
We could work out 25  11 which equals 14. There would be 14 buns left in this example.
Subtract the number of items eaten from the amount that Imani started with to find the answer.
Solution 14 buns had been eaten (36  14 = 22).
 17 apples had been eaten (29  17 = 12).
 24 icecreams had been eaten (51  24 = 27).
 34 packets of crisps had been eaten (67  34 = 33).

Find the missing numbers.
HintsIf the first number is missing, you can add the other two numbers to find the answer.
If the middle number is missing, you can subtract the answer from the first number to find the missing number.
Always check your answers by performing the inverse operation.
You could use part whole models like these to help.
Solution1.54  42 = 12
 To solve this we can add 42 + 12 = 54
 To solve this we can subtract 30 from 68: 68  30 = 38
 To solve this we can add 53 + 22 = 75
 To solve this we can add 21 + 66 = 87
 To solve this we can subtract 17 from 49: 49  17 = 32

Which number is missing?
HintsIf the first number is missing, what should you do with the other two numbers?
If the middle number is missing, where can you move the final number to?
Check your answers by performing the inverse operation.
For example, once you have worked out what you add to 21 to get 49 by solving 49  21 = ?, you can add 21 + ? to check it equals 49.
Solution28
 28  16 = 12
 49  28 = 21
 37  25 = 12
 70  37 = 33
 46  22 = 24
 68  46 = 22
 59  46 = 13

How many screws have Mr. Squeaks and Imani used?
HintsWhere could you move the 16 to, to help you solve this problem?
You could try using a part whole model like this to help find the answer.
Try working out 48  16 to find the correct answer.
You can check your answer by adding it to 16. If it makes 48 you have found the correct number.
SolutionMr. Squeaks and Imani have used 32 screws. 48  32 = 16.
We can work this out by solving 48  16, which equals 32. We can check our answer by adding 16 + 32, which equals 48.

How much energy did Imani start with today?
HintsThe first number is missing in this subtraction problem, so what do you need to do with the other numbers to find the answer?
You are trying to solve ?  28  13  31 = 18. What would the inverse operation of this be?
SolutionImani started with 90 units of energy today.
90  28  13  31 = 18
We can solve this by adding 18 + 31 + 13 + 28 to get 90.