Partitioning to twenty
You learn that every number from 11 to 19 is made of one ten and some ones.
How to work it out
- Partition the teen number into ten and ones: 14 is 10 and 4.
- Identify the given part; the other part is the answer.
A worked example
10 + _ = 14
- Partition 14 into 10 and 4.
- The 10 is already there.
- The missing part is 4, so 10 + 4 = 14.
More examples
6 + _ = 16
- Partition 16 into 10 and 6.
- The 6 is already there.
- The missing part is 10, so 6 + 10 = 16.
10 + _ = 19
- Partition 19 into 10 and 9.
- The 10 is already there.
- The missing part is 9, so 10 + 9 = 19.
Try it yourself
10 + • = 16
8 + • = 18
Watch out
You count from 6 to 16 and get 9 or 11.
Look at the number: 16 is 10 and 6. If the 6 is already there, the 10 is missing. You do not need to count.
Tips
- Only look at the last digit: that is how many ones there are.
- Learn the facts from 10 + 1 to 10 + 9 by heart.
Why it works
You can see it in 14: the 1 is a ten and the 4 is four ones. If you know that, you do not have to count. You need this later for sums like 8 + 6, where you cross ten.