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Maths · Addition & Subtraction

Missing-number problems

Solve equations with an unknown, like 27 + ? = 63.

Not startedCAMBRIDGE · 4CBSE · IVIB · PYP-4

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What is a missing-number problem?

Sometimes a number hides in a sum! We see a question mark or a box instead. Our job is to work out what the mystery number is. For example: 15 + ? = 22. We need to find what goes in place of the question mark.

How do we find the missing number?

Think about what the sum is asking. If 15 plus something equals 22, we can ask: what do we add to 15 to reach 22? We can count on from 15 to 22, or use subtraction. Take 22 and take away 15. That gives us 7, so the missing number is 7!

Using subtraction to help

Addition and subtraction are partners. They help each other out! If we know 27 + ? = 63, we can subtract 27 from 63. That tells us the missing number. 63 take away 27 equals 36. So the mystery number is 36.

Check your answer

Always check your work! Put your answer back into the original sum. Does it work? If 27 + 36 = 63, then 36 is correct. If the sum doesn't match, try again. Checking helps us learn.

Worked examples

  1. 1. Find the missing number: 45 + ? = 82

    1. We need to find what number added to 45 makes 82.
    2. Use subtraction to help: 82 – 45.
    3. 82 – 45 = 37.
    4. So the missing number is 37.
    5. Check: 45 + 37 = 82. It works!

  2. 2. Find the missing number: ? + 28 = 55

    1. Something plus 28 equals 55.
    2. Subtract 28 from 55 to find the mystery number.
    3. 55 – 28 = 27.
    4. The missing number is 27.
    5. Check: 27 + 28 = 55. Correct!

  3. 3. Find the missing number: 134 + ? = 200

    1. We add something to 134 to reach 200.
    2. Work out 200 – 134.
    3. 200 – 134 = 66.
    4. The missing number is 66.
    5. Check: 134 + 66 = 200. Perfect!

Try it yourself

Find the missing number: 38 + ? = 75