Topic 1 of 8
Addition and subtraction facts within 20 from memory
Step 4 · Place value to 1,000 topics
Know every sum of two one-digit numbers by heart, using doubles, making ten and facts you already know to fill the gaps.
Doubles and near doubles
A double adds a number to itself: 6 + 6 = 12, 8 + 8 = 16. Doubles are easy to remember because they come in pairs, like wheels on a bike or legs on a bird.
| Double | Sum |
|---|---|
| 1 + 1 | 2 |
| 2 + 2 | 4 |
| 3 + 3 | 6 |
| 4 + 4 | 8 |
| 5 + 5 | 10 |
| 6 + 6 | 12 |
| 7 + 7 | 14 |
| 8 + 8 | 16 |
| 9 + 9 | 18 |
A near double is one more than a double. 7 + 8 is 7 + 7 and 1 more, so 14 + 1 = 15.
Bridging through ten
When one number is close to 10, fill it up to 10 first. 9 + 6: 9 needs 1, so take 1 from the 6. 10 + 5 = 15.
- the 9
- the 6
For subtraction, go down to 10 first: 14 − 6 is 14 − 4 = 10, then 2 more, 10 − 2 = 8.
Using a fact you know
Every addition fact gives you subtraction facts for free. If you know 9 + 6 = 15, then 15 − 9 = 6 and 15 − 6 = 9.
Example: Think addition
Find 16 − 7 using a fact you know.
- Think: 7 + ? = 16.
- You know 7 + 9 = 16.
- So 16 − 7 = 9.
Answer9
Swapping the order also halves the facts to learn: if you know 3 + 8, you know 8 + 3.
Try it yourself
Work each one on paper first, then open the answer to check.
6 + 6
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12.
6 + 7
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13: one more than 6 + 6.
8 + 5
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13: 8 + 2 = 10, then 3 more.
15 − 7
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8, because 7 + 8 = 15.
9 + 9
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18.
13 − 4
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9: 13 − 3 = 10, then 1 more.