The idea, step by step
Parts and a whole
In an addition, the two addends are parts and the answer is the whole. If a part is missing, subtract the known part from the whole. For ? + 258 = 700, find 700 − 258.
In a subtraction, the starting number is the whole. If the start is missing, add the result and the amount removed. For ? − 186 = 329, find 329 + 186. A part–whole drawing can help you decide which value is missing.
The unknown's position matters
In 904 − ? = 487, the unknown is the amount removed. Find the difference between the start and finish: 904 − 487. Do not choose an operation just by looking at the sign.
Put your result back into the original statement. Both sides must have exactly the same value. The equals sign is not an instruction to put an answer on the right: 56 = ? + 19 is also a valid equation.
A common trap
For ? − 46 = 82, calculating 82 − 46.
Instead: The missing value is the starting whole. Add the removed part back: 82 + 46 = 128, and 128 − 46 = 82.
See the thinking
Find a missing addend
Find the missing number in ? + 258 = 700.
- 700 is the whole and 258 is one known part.
- 700 − 258 = 442.
- Check: 442 + 258 = 700.
Find the amount removed
Find the missing number in 904 − ? = 487.
- The start is 904 and the finish is 487.
- 904 − 487 = 417.
- Check: 904 − 417 = 487.
Check your understanding
Find the missing number: 650 = ? + 275.
Show answer
375.
650 − 275 = 375. Substitution gives 650 = 375 + 275.
Your turn
6 questions · 8 marks · Work at your own pace.
Read the questions online
- Find ?: ? + 147 = 500.1 mark
- Find ?: 368 + ? = 720.1 mark
- Find ?: ? − 245 = 318.1 mark
- Find ?: 900 − ? = 427.1 mark
- Find ?: 156 + 84 = ? + 90.2 marks
- A school wants 800 tickets. It has printed 365. Write a missing-value equation and find how many more to print.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Find ?: ? + 147 = 500.1 mark
500 − 147 = 353.
353 + 147 = 500.
353. - Find ?: 368 + ? = 720.1 mark
720 − 368 = 352.
368 + 352 = 720.
352. - Find ?: ? − 245 = 318.1 mark
318 + 245 = 563.
563 − 245 = 318.
563. - Find ?: 900 − ? = 427.1 mark
900 − 427 = 473.
900 − 473 = 427.
473. - Find ?: 156 + 84 = ? + 90.2 marks
The left side is 240.
240 − 90 = 150, so both sides equal 240.
150. - A school wants 800 tickets. It has printed 365. Write a missing-value equation and find how many more to print.2 marks
800 − 365 = 435.
Check: 365 + 435 = 800.
365 + ? = 800; 435 more tickets.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Place-value revision; odd/even properties; addition/subtraction strategies; missing values; rounding/checking.
Official curriculum source ↗