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
The lesson
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.
Addition and subtraction facts are partners. If you know 8 + 4 = 12, you already know 12 − 8 = 4 and 12 − 4 = 8, without counting at all.
Bridge through 10 to add or subtract, or use a fact you already know.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 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.