Understand the idea.
Line up matching places
Keep matching places together: hundreds with hundreds, tens with tens and ones with ones. In , the hundreds make , the tens make and the ones make , giving .
In columns, line up the rightmost digits first. A two-digit number has no hundreds, so its tens digit belongs beneath the tens, not the hundreds. You may use a zero to mark the empty hundreds place.
Subtract by place value
Subtract a number in parts without changing its value. For , remove , then , then . Keep a running total after each removal so no part is missed or removed twice.
Check that your answer fits the situation. Subtracting a positive amount makes the starting number smaller. You can also add the removed amount back to your answer; it should recover the original number.
See the method in action.
Add two three-digit numbers
Calculate 235 + 142.
Worked solution
- 1
Combine the hundreds.
Both digits in this place count hundreds.
- 2
Combine the tens.
Three tens and four tens make seven tens.
- 3
Combine the ones.
The ones total is less than ten, so no exchange is needed.
- 4
Rebuild the total.
The partial sums include every place once.
377
Subtract a two-digit number
Calculate 468 − 35.
Worked solution
- 1
Split the amount to subtract.
Thirty-five is three tens and five ones, with no hundreds.
- 2
Remove the three tens.
Only the tens change in this step.
- 3
Remove the five ones.
Both parts of thirty-five have now been removed.
- 4
Check the difference.
Adding back the removed amount returns to the starting whole.
433
Try it yourself.
Check your understanding
Calculate 421 + 56.
Show answer & reasoning
477
Split into : , then . The two parts together add exactly .
Keep practising
6 worksheet questions
Read the questions online
- Calculate .
- Calculate .
- Calculate .
- Calculate .
- A class reads 213 pages on Monday and 154 on Tuesday. How many pages altogether?
- A library has 786 books. It lends out 243. How many remain?
Read the worked solutions
- Calculate .
; ; .
567 - Calculate .
; add 7.
489 - Calculate .
; ; .
453 - Calculate .
; .
533 - A class reads 213 pages on Monday and 154 on Tuesday. How many pages altogether?
.
367 pages - A library has 786 books. It lends out 243. How many remain?
; ; .
543 books
Curriculum reference & lesson scope
Australian Curriculum v9 / SA Curriculum Mathematics
Mental strategies; two-/three-digit calculations; regrouping; inverse operations; missing values.
Official curriculum source ↗