Counting back to ten
You learn how much to take away from a number between 10 and 20 to get to exactly ten.
How to work it out
- Split the teen number into ten and ones: 16 is 10 and 6.
- Subtract the ones to land exactly on ten.
A worked example
16 − _ = 10
- Partition 16 into 10 and 6.
- Take away the 6 ones.
- So 16 − 6 = 10.
More examples
13 − _ = 10
- Partition 13 into 10 and 3.
- Take away the 3 ones.
- So 13 − 3 = 10.
19 − _ = 10
- Partition 19 into 10 and 9.
- Take away the 9 ones.
- So 19 − 9 = 10.
Try it yourself
14 − • = 10
18 − • = 10
Watch out
You take away 10, because you want to get to ten: for 16 − _ = 10 you say 10.
Ten is where you end up, not what you take away. You take away the ones after the 1.
Tips
- Only look at the digit after the 1: that is how much you take away.
Exceptions
- For 10 − _ = 10 you take away nothing.
Why it works
A number like 16 is one ten and 6 ones. Take away the ones and the ten is left. You need this later when you subtract across ten: you do 16 − 9 as 16 − 6 − 3.