Tens Complement
What Makes 10?
The "Tens Complement" is the number we add to make 10:
Example: The Tens Complement of 3 is 7, because 3+7=10
Here are all the Tens Complements:
| Number | Tens Complement | Why? Because: |
|---|---|---|
| 0 | 10 | 0 + 10 = 10 |
| 1 | 9 | 1 + 9 = 10 |
| 2 | 8 | 2 + 8 = 10 |
| 3 | 7 | 3 + 7 = 10 |
| 4 | 6 | 4 + 6 = 10 |
| 5 | 5 | 5 + 5 = 10 |
| 6 | 4 | 6 + 4 = 10 |
| 7 | 3 | 7 + 3 = 10 |
| 8 | 2 | 8 + 2 = 10 |
| 9 | 1 | 9 + 1 = 10 |
| 10 | 0 | 10 + 0 = 10 |
But you only have to remember half the table!
1 + 9 = 10 and 9 + 1 = 10
1 and 9 are partners!
So are 2 and 8, 3 and 7, 4 and 6 and 5 and 5
Why is this Important?
Knowing the Tens Complements makes some mental arithmetic easier.
Example: 25 + 8
Start with:25 + 8
8 and 2 make 10, so:25 + 10 − 2
Add the 10:35 − 2
Subtract the 2:33
In practice we think quickly like this:
"8 is 2 away from 10": 25+10 is 35, −2 is 33
Example: 74 − 9
"9 is 1 away from 10": 74−10 is 64, +1 is 65
It is also used in Quick Subtraction.
Practice!
You can practice by using What Makes 10?