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subtraction

Missing number subtraction

Solve □ − 5 = 8 and 14 − □ = 6 with models.

Quick answer

Missing number subtraction means finding the number that goes in the box to make a subtraction sentence true. For example, in □ − 5 = 8, the missing number is 13, because 13 − 5 = 8.

Learning outcome

By the end of this page, you will be able to find the missing number in subtraction problems like □ − 5 = 8 or 14 − □ = 6 using models, counting, and number facts. Success signs:
  • You can solve subtraction sentences with a missing number
  • You can use objects or drawings to help you find the answer
  • You can check if your answer makes the sentence true
  • You can explain how you found the missing number

Why this matters

Missing number subtraction helps you understand how numbers work together. It's useful when you're solving puzzles, playing games, or working out how many things you have left. It's also a big step towards understanding algebra later on. You already use this kind of thinking when you wonder, “If I had 10 cookies and ate some, how many are left?”

How it works

To find the missing number in a subtraction sentence, think about what number you need to subtract from or take away from to get the answer. Method 1: Use a model or objects You can use blocks, counters, or draw pictures to show the problem. Method 2: Use addition to check If you have □ − 5 = 8, think: What number minus 5 gives me 8? You can add 5 to 8 to find the answer. Method 3: Think backwards If you know the answer and one part of the subtraction, you can work backwards to find the missing number.

Examples you can see

Let’s try a few examples to see how this works. Example 1: □ − 5 = 8 Imagine you have some apples. You give away 5 apples and you have 8 left. How many apples did you start with? You can draw 8 apples and then add 5 more to see how many you had at first. Or you can think: 8 + 5 = 13. So, □ = 13. Example 2: 14 − □ = 6 Here, you start with 14 apples. You eat some apples and have 6 left. How many did you eat? You can draw 14 apples, take away 6, and count how many you took away. Or you can think: 14 − 6 = 8. So, □ = 8. Example 3: 9 − □ = 3 You start with 9 pencils and end with 3. How many did you give away? Draw 9 pencils, take away 3, and count how many you took. Or think: 9 − 3 = 6. So, □ = 6.

Try it as a mini adventure

Adventure: The Lost Treasure Hunt You are on a treasure hunt! You have a map that says, “Start with some gold coins. Spend 7 coins on a bridge. You have 12 coins left. How many did you start with?” This is a missing number subtraction problem! Step 1: Draw 12 coins. Step 2: Add 7 more coins to show what you spent. Step 3: Count all the coins together. You have 19 coins at the start. So, the problem is: □ − 7 = 12 The answer is □ = 19. Try solving this one: Step 1: You start with 15 sweets. Step 2: You give some to your friend. Step 3: You have 9 left. How many did you give away? Answer: 15 − 9 = 6 So, you gave away 6 sweets. Problem: 15 − □ = 9 Answer: □ = 6

Common mistakes

  • Mistake: Thinking that □ − 5 = 8 means you take away 8 from 5.
    Why it happens: Confusing the order of numbers.
    Fix: Remember, you are looking for the number that when you subtract 5, you get 8. So, add 5 to 8 to find the answer.
  • Mistake: Using subtraction instead of addition to solve □ − 5 = 8.
    Why it happens: Not knowing that addition can help find missing numbers.
    Fix: Think: What number plus 5 gives me 8? That’s 8 − 5 = 3. But wait — that’s wrong! You want the number that, when you take away 5, gives 8. So it’s 8 + 5 = 13.
  • Mistake: Thinking that in 14 − □ = 6, the answer is 14 + 6.
    Why it happens: Mixing up which operation to use.
    Fix: Remember, you are looking for the number that when subtracted from 14 gives 6. So, 14 − 6 = 8.

Parent and carer guide

How to help your child:
  • Use real objects like counters, blocks, or toys to show the problem.
  • Ask your child to explain how they found the answer.
  • Encourage them to check their answer by substituting it back into the original problem.
  • Play games where they have to find missing numbers in subtraction sentences.
  • Don’t rush — let them think through each step.
What to watch for:
  • Confusion with the order of numbers in subtraction.
  • Using the wrong operation to solve the problem.
  • Not checking if the answer makes sense.
What not to worry about:
  • They might get the answer wrong at first — that’s part of learning!
  • They may need to count or draw to help them understand.
  • They might use a different method — as long as it works, it’s okay.

Practice questions

  1. □ − 4 = 9
  2. 15 − □ = 7
  3. □ − 3 = 11
  4. 12 − □ = 5
  5. □ − 6 = 10
  6. 18 − □ = 9
  7. □ − 2 = 15
  8. 10 − □ = 3
  9. □ − 8 = 4
  10. 16 − □ = 8
Answers:
  1. □ = 13
  2. □ = 8
  3. □ = 14
  4. □ = 7
  5. □ = 16
  6. □ = 9
  7. □ = 17
  8. □ = 7
  9. □ = 12
  10. □ = 8

FAQ

What is a missing number subtraction problem?

It’s a subtraction sentence where one number is missing and you have to find it. For example, in □ − 5 = 8, you need to find what number goes in the box.

How can I solve missing number subtraction?

You can use objects, draw pictures, or think of addition to find the missing number. For example, in □ − 5 = 8, you can think: 8 + 5 = 13.

Why is this important?

It helps you understand how numbers work and prepares you for more complex maths like algebra later on.

Can I use a number line to solve this?

Yes! You can start at the answer and count backwards or forwards to find the missing number.

What if I get it wrong?

That’s okay! Mistakes help you learn. Try again and check your answer to see if it works.