Subtraction bridging ten
You learn subtractions that cross ten by going back to exactly ten first.
How to work it out
- Split the second number to land on ten.
- Subtract to ten first.
- Then subtract the rest.
A worked example
15 − 7
- Split 7 into 5 and 2, because 15 − 5 = 10.
- Take away the 5 first: 10.
- Then take away the 2: 10 − 2 = 8.
More examples
13 − 8
- Split 8 into 3 and 5, because 13 − 3 = 10.
- Take away the 3 first: 10.
- Then take away the 5: 10 − 5 = 5.
12 − 4
- Split 4 into 2 and 2, because 12 − 2 = 10.
- Take away 2 first: 10.
- Then take away the other 2: 10 − 2 = 8.
Try it yourself
16 − 9 =
15 − 7 =
Watch out
You swap the ones round: for 13 − 8 you do 8 − 3 = 5 and say 15.
With a minus, you take the second number away from the first. Go back to 10 first, and it always works.
Tips
- Look at the ones in the number you start with: take those away first.
- Check with an addition: 8 + 7 = 15, so 15 − 7 = 8.
Why it works
From a number like 15 it is easy to get back to ten: take away the 5 ones. After that, taking away from 10 is not hard either. That way 15 − 7 becomes two small steps: 15 − 5 and 10 − 2.