Why 10 is special
Our number system is built around tens. That's why adding to or from 10 is easier than other sums.
10 is the friendly number.
If we can land on 10 first, the rest is easy.
Most KS1 sums that feel hard are sums that cross over 10. Like 8 + 5, or 7 + 6. We're going to learn the trick that makes those easy.
The trick: split the smaller number
To do 8 + 5, here's the trick:
Step 1: How many do I need to make 10?
8 needs 2 more to make 10.
Step 2: Split the 5 into 2 and 3.
Step 3: 8 + 2 = 10, then 10 + 3 = 13!
8 + 5 = 8 + 2 + 3 = 10 + 3 = 13
Worked example: 7 + 6
Let's try another one. 7 + 6.
- 7 needs 3 more to make 10.
- Split 6 into 3 and 3.
- 7 + 3 = 10.
- 10 + 3 = 13!
7 + 6 = 7 + 3 + 3 = 10 + 3 = 13
Worked example: 9 + 4
Now 9 + 4. This one is easier because 9 is so close to 10.
- 9 needs just 1 more to make 10.
- Split 4 into 1 and 3.
- 9 + 1 = 10.
- 10 + 3 = 13!
9 + 4 = 9 + 1 + 3 = 10 + 3 = 13
Did you notice all three examples gave 13? That's because 7+6, 8+5, 9+4 all equal 13!
Subtraction works too
The same trick helps with subtraction. To do 13 − 5:
- Subtract enough to land on 10. 13 − 3 = 10.
- You've used 3 of the 5. Two more to go.
- 10 − 2 = 8!
13 − 5 = 13 − 3 − 2 = 10 − 2 = 8
Quick check!
Try these in your head using the bridging trick:
8 + 6 = ?
9 + 7 = ?
14 − 6 = ?
15 − 8 = ?
Answers: 14 · 16 · 8 · 7. Got them all? Brilliant — try the quiz next. Got some wrong? Watch the lesson again and try the worksheet.
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