Topic 6 of 8
Step 3: Adding and subtracting to 20
Adding within 100
Step 3 · Adding and subtracting to 20 topics
Add and take away tens, add ones to a two-digit number, and find ten more or ten less in your head.
Adding and taking away tens
Adding tens is like adding ones, but counting tens: 3 tens + 4 tens = 7 tens, so 30 + 40 = 70.
Taking away tens works the same way: 8 tens − 5 tens = 3 tens, so 80 − 50 = 30.
To add tens to any two-digit number, add to the tens digit only: 46 + 30 = 76, because 4 tens + 3 tens = 7 tens and the 6 ones stay.
Adding ones to a two-digit number
To add ones, add them to the ones digit: 52 + 4 = 56. When the ones make 10 or more, they make a new ten.
Example: Making a new ten
Work out 38 + 5.
- 8 ones and 5 ones make 13 ones.
- 13 ones is 1 ten and 3 ones.
- 3 tens and 1 more ten is 4 tens, with 3 ones.
Answer43
Making ten works here too: 38 + 2 = 40, then 3 more makes 43.
Ten more and ten less
Ten more than a number has one more ten and the same ones: ten more than 64 is 74. Ten less has one fewer ten: ten less than 64 is 54.
| Ten less | Number | Ten more |
|---|---|---|
| 17 | 27 | 37 |
| 54 | 64 | 74 |
| 80 | 90 | 100 |
Try it yourself
Work each one on paper first, then open the answer to check.
40 + 30
Show answer
70.
90 − 60
Show answer
30.
53 + 20
Show answer
73.
47 + 6
Show answer
53. 47 + 3 = 50, then 3 more.
Ten more than 85
Show answer
95.
Ten less than 32
Show answer
22.