Adding: the box is a part
3,418 + □ = 3,905
- 3,905 is the whole. 3,418 is one part, and the box is the other part.
- To find a missing part, take the known part away from the whole: 3,905 − 3,418 = 487.
- Check: 3,418 + 487 = 3,905. ✓
Answer: 487
Year 6 · Arithmetic skills · Skill 3A
Skill 3A: Missing numbers and inverse operations
Use inverse operations to find the missing number in addition and subtraction calculations.
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Each file is an editable Word document (.docx) with 25 questions in five columns. It opens in Word, Google Docs, Pages and LibreOffice, so you can change questions or add working space before printing.
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Worked examples
A missing number problem is a normal calculation with one number hidden. Use the inverse (opposite) operation to find it, then check by putting it back.
Subtracting: the box is the start
□ − 586 = 2,114
- The box is the number we started with, before 586 was taken away and 2,114 was left.
- Reverse it: add 586 back on. 2,114 + 586 = 2,700.
- Check: 2,700 − 586 = 2,114. ✓
Answer: 2,700
Subtracting: the box is the amount taken away
7,300 − □ = 7,129
- We started with 7,300 and ended with 7,129. The box is how much was taken away.
- The amount taken away is the difference: 7,300 − 7,129 = 171.
- Check: 7,300 − 171 = 7,129. ✓
Answer: 171
Top tips
- Decide first: is the box a part, the start, or the amount taken away?
- A bar model with the whole on top makes this clear.
- Always put your answer back in the box to check.
Watch out for
- Adding when you should subtract, or the other way round.
- For “□ − 586 = 2,114”, subtracting 586 from 2,114 instead of adding it.
Try some questions
These are from the worksheet. Work each one out, then open it to check.
- 4,532 + □ = 4,658
Show the answer
126
- 8,521 + □ = 8,897
Show the answer
376
- 8,782 − □ = 8,315
Show the answer
467
- 5,003 + □ = 5,279
Show the answer
276