Addition within twenty
You learn additions to twenty where you leave the ten alone and only work with the ones.
How to work it out
- Keep the ten.
- Add the ones together.
- Put the ten back in front.
A worked example
13 + 5
- Leave the 10 in 13 alone.
- Add the ones: 3 + 5 = 8.
- Put the 10 back: 10 + 8 = 18.
More examples
12 + 6
- Leave the 10 in 12 alone.
- Add the ones: 2 + 6 = 8.
- Put the 10 back: 10 + 8 = 18.
4 + 11
- Leave the 10 in 11 alone.
- Add the ones: 4 + 1 = 5.
- Put the 10 back: 10 + 5 = 15.
Try it yourself
12 + 6 =
13 + 5 =
Watch out
You forget to put the ten back: for 13 + 5 you say 8.
There was already a ten in 13, and it does not go away. The answer is more than 10.
Tips
- Leave the ten alone: only work with the ones.
- You can swap them round: 4 + 11 is the same as 11 + 4.
Exceptions
- If the ones add up to 10 or more, like 17 + 5, you cross ten. Then use Make ten first.
Why it works
13 is one ten and 3 ones. If you add 5, the ten does not change: only the ones go up. That way you work with small numbers you already know, and then put the ten back in front.