The idea, step by step
Line up matching places
When recording a calculation, ones go under ones, tens under tens and hundreds under hundreds. For 324 + 153, combine 300 + 100, 20 + 50 and 4 + 3.
The sum is 400 + 70 + 7 = 477. Saying each place helps you see what each digit means. A two-digit number has no hundreds, so leave that hundreds place empty or use a zero.
Subtract by place value
For 586 − 243, remove 200, then 40, then 3. This gives 386, 346, then 343. You may record the same thinking in columns.
Estimate before accepting your answer. About 600 − 200 is about 400, so 343 is a possible exact answer. A result larger than 586 would not fit this subtraction.
A common trap
Putting the 3 from 35 under the hundreds digit of 468.
Instead: The 3 represents 3 tens. Line it up with the tens digit, 6.
See the thinking
Add two three-digit numbers
Calculate 235 + 142.
- Add hundreds: 200 + 100 = 300.
- Add tens: 30 + 40 = 70.
- Add ones: 5 + 2 = 7.
- Combine: 300 + 70 + 7 = 377.
Subtract a two-digit number
Calculate 468 − 35.
- Treat 35 as 0 hundreds, 3 tens and 5 ones.
- 468 − 30 = 438.
- 438 − 5 = 433.
Check your understanding
Calculate 421 + 56.
Show answer
477
421 + 50 = 471, then add 6.
Your turn
6 questions · Work at your own pace.
Read the questions online
- Calculate 326 + 241.
- Calculate 452 + 37.
- Calculate 684 − 231.
- Calculate 597 − 64.
- 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 326 + 241.
300 + 200 = 500; 20 + 40 = 60; 6 + 1 = 7.
567 - Calculate 452 + 37.
452 + 30 = 482; add 7.
489 - Calculate 684 − 231.
684 − 200 = 484; −30 = 454; −1 = 453.
453 - Calculate 597 − 64.
597 − 60 = 537; −4 = 533.
533 - A class reads 213 pages on Monday and 154 on Tuesday. How many pages altogether?
213 + 154 = 367.
367 pages - A library has 786 books. It lends out 243. How many remain?
786 − 200 = 586; −40 = 546; −3 = 543.
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 ↗