Complements of 20
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Complements of 20: skill guide
| Objective | Every problem fills two complete ten strips with 20 dots divided into a red part and a green part. |
|---|---|
| Grade | Grade 1 |
| Standard | CCSS.MATH.CONTENT.1.OA.C.6 |
| Difficulty | Developing · Abstract |
| Representation | Numerals |
| Practice format | Interactive, account-free practice with immediate feedback |
Objective
Every problem fills two complete ten strips with 20 dots divided into a red part and a green part.
What this practice covers
Every problem fills two complete ten strips with 20 dots divided into a red part and a green part. Learners identify the green complement that joins the red dots to make 20. The model supports several strategies: count on from the red amount, subtract from 20, use ten as a landmark, or recognize a familiar number pair.
Standards alignment
Supports Common Core: CCSS.MATH.CONTENT.1.OA.C.6 — Add and subtract within 20 using strategies such as making ten and relating addition to subtraction.
Who it’s for
It is intended for kindergarten learners who recognize teen quantities and are beginning to reason about parts of 20. The complete two-strip model keeps the whole visible while the colors distinguish the two parts.
What to know first
- Learners should count through twenty, understand that the two strips always contain 20 spaces, and have some experience finding pairs that make 10. Automatic addition or subtraction recall is not required.
Difficulty and scaffolding
Developing · Abstract stage
Problem representations
- Numerals
Three worked examples
- If 12 dots are red, 8 are green because 12 + 8 = 20.
- If 10 dots are red, the other full strip has 10 green dots: 10 + 10 = 20.
- If all 20 dots are red, 0 green dots are needed.
Likely misconceptions
- Learners may answer with the red amount, count a divider as a dot, or stop after the first strip. Read the color named in the question and count only circles. The final check is always red plus green equals 20.
Hints and explanations
- If 12 dots are red, 8 are green because 12 + 8 = 20.
- If 10 dots are red, the other full strip has 10 green dots: 10 + 10 = 20.
- If all 20 dots are red, 0 green dots are needed.
What to practice next
Build teen numbers with tens and ones, then solve missing-addend equations such as 13 + ? = 20. The same complement idea later supports mental addition and subtraction across a multiple of ten.
Editorial review
Reviewed by Brilliant’s curriculum team. Exercises are generated within defined difficulty ranges and checked against verified answer rules.
Complements of 20 FAQs
What is a complement of 20?
It is the amount that joins another amount to make 20. For example, 7 and 13 are complements because 7 + 13 = 20.
How can ten help with complements of 20?
Use the boundary between the strips as a landmark. First determine how much reaches 10, then consider the second group of 10. This breaks a larger relationship into familiar parts.
Why use two colors?
The two colors show both parts while preserving one complete whole of 20. Learners can see that as one part grows, the other must shrink by the same amount.
Can subtraction be used?
Yes. If 14 dots are red, calculate 20 − 14 = 6 to find the green complement. The model shows why that subtraction and 14 + 6 = 20 describe the same relationship.