The idea, step by step
Ask if the size makes sense
A check helps you notice an answer that is too large or too small. 41 + 28 should be close to 40 + 30 = 70. An answer of 19 cannot be right because adding 28 makes 41 larger, not smaller.
Undo or try another way
If 36 + 17 = 53, check with 53 − 17 = 36. If 62 − 24 = 38, check with 38 + 24 = 62. Repeating the same mistaken steps is less useful than a different method.
A common trap
Saying an estimate proves an exact answer.
Instead: An estimate checks the size. Use an exact method or inverse calculation to check the exact answer.
See the thinking
Undo an addition
Check 27 + 15 = 42.
- Subtract one added part from the total: 42 − 15.
- 42 − 10 − 5 = 27, which is the other part.
Spot a wrong subtraction
A student writes 54 − 21 = 75. Is this correct?
- Subtracting a positive amount should make 54 smaller, but 75 is larger.
- Calculate 54 − 20 − 1 = 33; check 33 + 21 = 54.
Check your understanding
Check 48 − 16 = 32 using addition.
Show answer
32 + 16 = 48
The remaining part and removed part make the starting whole.
Your turn
6 questions · Work at your own pace.
Read the questions online
- Check 24 + 13 = 37 using subtraction.
- Check 45 − 12 = 33 using addition.
- Is 32 + 26 = 58 correct? Show a different calculation.
- A student writes 67 − 20 = 65. Fix the answer.
- Would 86 be a sensible answer for 29 + 18? Explain.
- Find and check 53 − 28.
Read the worked solutions
- Check 24 + 13 = 37 using subtraction.
Undo the addition by removing one part.
37 − 13 = 24 (or 37 − 24 = 13). - Check 45 − 12 = 33 using addition.
Add the removed amount back.
33 + 12 = 45. - Is 32 + 26 = 58 correct? Show a different calculation.
Combine tens and ones separately.
Yes. 30 + 20 + 2 + 6 = 58. - A student writes 67 − 20 = 65. Fix the answer.
Subtract 2 tens from 6 tens; the 7 ones stay unchanged.
47 - Would 86 be a sensible answer for 29 + 18? Explain.
The exact total is 47, far less than 86.
No. The answer should be close to 30 + 20 = 50. - Find and check 53 − 28.
53 − 20 = 33; 33 − 8 = 25.
25; check 25 + 28 = 53.
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 ↗