The idea, step by step
Add the parts
For 34 + 23, combine 3 tens and 2 tens to get 5 tens. Combine 4 ones and 3 ones to get 7 ones. The answer is 57. Drawing tens and ones can keep the parts clear.
Choose a short route
When adding a small number, count on from the larger number. When subtracting, count back or find the difference. For 52 − 49, counting on from 49 is shorter than making 49 backward jumps.
A common trap
Subtracting the smaller digit from the larger digit in every column.
Instead: Subtraction has an order. In 52 − 28, you need to subtract 28 from 52, not swap the ones. Use a number line or regroup a ten.
See the thinking
Add two two-digit numbers
Find 46 + 21.
- 46 + 20 = 66.
- 66 + 1 = 67.
Find a small difference
Find 62 − 58.
- Count from 58 to 60: 2.
- Count from 60 to 62: 2 more; 2 + 2 = 4.
Check your understanding
Find 35 + 12.
Show answer
47
35 + 10 = 45, then add 2.
Your turn
6 questions · Work at your own pace.
Read the questions online
- Find 28 + 4.
- Find 43 + 25.
- Find 76 − 30.
- Find 54 − 21.
- There are 38 children inside and 7 outside. How many children are there?
- A ribbon is 51 cm long. Another is 48 cm long. How much longer is the first?
Read the worked solutions
- Find 28 + 4.
28 + 2 = 30, then add 2 more.
32 - Find 43 + 25.
43 + 20 = 63; 63 + 5 = 68.
68 - Find 76 − 30.
Take 3 tens from 7 tens; keep the 6 ones.
46 - Find 54 − 21.
54 − 20 = 34; 34 − 1 = 33.
33 - There are 38 children inside and 7 outside. How many children are there?
38 + 2 + 5 = 45.
45 children. - A ribbon is 51 cm long. Another is 48 cm long. How much longer is the first?
48 to 50 is 2 cm; 50 to 51 is 1 cm; 2 + 1 = 3 cm.
3 cm.
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 ↗