Operations & Algebraic Thinking
Fluently Adding & Subtracting Within 20
Building speed and accuracy with sums and differences within 20.
Building Speed with Facts You Know
You already know how to add and subtract within 20 by counting on or counting back. Fluency means getting fast and accurate at those same facts, so you can use them instantly inside bigger problems without stopping to count every time.
Definition — Fluency
One way to build fluency is to notice relationships between facts instead of memorizing each one separately. Knowing 8 + 8 = 16 makes 8 + 9 easy — it's just one more, so 8 + 9 = 17. Making a ten is another shortcut: 9 + 6 is the same as 10 + 5, since one unit moved from the 6 to the 9.
Worked Example — Using a Doubles Fact
Worked Example — Making a Ten
Tip
Common Mistakes
Falling back to counting one by one on every fact, even ones already practiced many times.
Pause and try to recall the fact from memory first. If it doesn't come quickly, use a strategy like making a ten instead of counting from 1.
Making a ten by moving the wrong amount, such as taking 2 from a 6 to add to a 9 instead of exactly 1.
Move only enough to make the first number reach 10 exactly. Since 9 needs 1 more to reach 10, move exactly 1 from the other number.
Key Takeaways
- Fluency means recalling addition and subtraction facts within 20 quickly and accurately.
- Doubles facts and making a ten are strategies that turn an unfamiliar fact into a familiar one.
- Fluent facts within 20 make larger addition and subtraction problems much faster to solve.
Summary
Fluency with facts within 20 makes every larger calculation faster and more reliable. The next topic builds toward multiplication, starting with a new way of thinking about groups of objects.
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