Addition and subtraction within 20
Add and subtract within 20 using strategies such as making ten, decomposing a number leading to ten, and using known facts
What a learner can do afterwards
- Solve 8 + 6 using making ten: 8 + 2 + 4 = 14
- Solve 13 − 4 by decomposing: 13 − 3 − 1 = 9
- Use a known fact (8 + 4 = 12) to derive 12 − 8 = 4
1 · Read
You already know your facts up to 10, like 8 + 2 or 6 − 3. Now you can use those same facts to solve bigger problems, all the way up to 20. The trick is to build a bridge through 10.
Mia has 13 stickers and gives 4 to her friend. She takes away 3 first to land on 10, then 1 more to land on 9. So 13 − 4 = 9.
A fact family uses the same three numbers in addition and subtraction. For example, 6 + 5 = 11, 5 + 6 = 11, 11 − 6 = 5, and 11 − 5 = 6. Each equation links the same numbers.
Bridge through 10 to add or subtract, or use a fact you already know.
Imported from hoofer-content corpus; per-item provenance in file. OpenStax-derived readings adapted from Prealgebra by Lynn Marecek and MaryAnne Anthony-Smith. Publisher terms for these readings are unknown.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
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.