Fluent adding and subtracting within 20
Fluently add and subtract within 20 using mental strategies and recall single-digit addition facts.
What a learner can do afterwards
- Answer any single-digit addition fact within 3 seconds
- Recall subtraction facts within 20 from memory (e.g. 15 − 8 = 7)
- Use known addition facts to derive related subtraction facts rapidly
1 · Read
Fluent means you know a fact quickly, in about 3 seconds, without counting. Counting on fingers can still give the right answer, but it is slow, so that fact is not fluent yet. The tricks below help a slow fact become quick.
A near double sits next to a double you know. For 5 + 6, think of 5 + 5 = 10. 6 is 1 more than 5, so 5 + 6 is 1 more than 10. That makes 5 + 6 = 11.
To make a ten, fill up to 10 first. For 9 + 3, 9 needs 1 more to make 10. Split 3 into 1 and 2. Then 9 + 1 = 10, and 10 + 2 = 12. So 9 + 3 = 12.
A fact family uses the same three numbers. Zoe knows 5 + 9 = 14. Without counting, she also knows 14 - 9 = 5 and 14 - 5 = 9. One addition fact you know gives you two subtraction facts.
To take away, you can stop at 10 first. For 11 - 3, split 3 into 1 and 2. Take away 1 to land on 10, then take away the other 2: 10 - 2 = 8. So 11 - 3 = 8.
You can also turn a take-away fact into an adding question. For 11 - 5, ask: 5 plus what makes 11? You know 5 + 6 = 11, so 11 - 5 = 6. Practise each fact until you just know it.
Know it, don't count it: use doubles, make-a-ten, and fact families to answer in a snap.
Original practice and reading text authored for the os-taxonomy project.
2 · Watch
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18 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.