The idea, step by step
Parts and a whole
In 5 + 8 = 13, the parts are 5 and 8 and the whole is 13. Swapping the two parts still gives 13. Taking either part from the whole leaves the other part.
| Addition | Subtraction |
|---|---|
| 5 + 8 = 13 | 13 − 5 = 8 |
| 8 + 5 = 13 | 13 − 8 = 5 |
Use the correct starting number
A subtraction fact starts with the whole. The related subtraction for 12 + 7 = 19 is 19 − 12 = 7, not 12 − 19. When the two parts match, some written facts repeat.
A common trap
Writing 4 − 13 = 9 as a related fact.
Instead: For this family, subtraction begins with the whole, 13. Write 13 − 4 = 9.
See the thinking
Use a whole and two parts
Write a fact family using 9, 4 and 13.
- 9 and 4 join to make 13.
- Use both orders for addition, then take each part from 13.
Use a known two-digit fact
You know 32 + 15 = 47. Find 47 − 32.
- 47 is the whole.
- Removing the part 32 leaves the other part, 15.
Check your understanding
Use 8 + 7 = 15 to find 15 − 8.
Show answer
7
Removing one part leaves the other.
Your turn
6 questions · Work at your own pace.
Read the questions online
- Complete: 6 + 9 = 15, so 9 + 6 = ___.
- Use 11 + 5 = 16 to find 16 − 5.
- Write one subtraction fact for 7 + 4 = 11.
- Use 23 + 14 = 37 to find 37 − 23.
- Write two addition facts using 8, 12 and 20.
- Tom writes 10 − 6 = 4 and says 6 + 4 = 10 is related. Explain why.
Read the worked solutions
- Complete: 6 + 9 = 15, so 9 + 6 = ___.
Swapping the parts does not change the whole.
15 - Use 11 + 5 = 16 to find 16 − 5.
Taking away 5 leaves the other part.
11 - Write one subtraction fact for 7 + 4 = 11.
The whole is 11; remove either part.
For example, 11 − 7 = 4. - Use 23 + 14 = 37 to find 37 − 23.
The other part is 14.
14 - Write two addition facts using 8, 12 and 20.
Both facts join the same two parts.
8 + 12 = 20 and 12 + 8 = 20. - Tom writes 10 − 6 = 4 and says 6 + 4 = 10 is related. Explain why.
Subtraction undoes the joining shown by addition.
Both use the parts 6 and 4 and the whole 10.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Facts within 20; one-/two-digit calculations; partitioning; related facts; missing numbers; checking.
Official curriculum source ↗