Missing addends to make 20
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Missing addends to make 20: skill guide
| Objective | Learners find the missing number that makes 20 when the first number is between 10 and 19, using numerals only. |
|---|---|
| Grade | Grade 1 |
| Standard | CCSS.MATH.CONTENT.1.OA.D.8 |
| Difficulty | Developing · Abstract |
| Representation | Numerical equation |
| Practice format | Interactive, account-free practice with immediate feedback |
Objective
Learners find the missing number that makes 20 when the first number is between 10 and 19, using numerals only.
What this practice covers
Each problem is an equation such as 16 + □ = 20, written with numerals only. The first number is from 10 to 19, and the learner fills the blank by choosing a number from 0 to 10. Learners can count on from the first number to 20, or find how many the ones digit needs to make 10.
Standards alignment
Supports Common Core: CCSS.MATH.CONTENT.1.OA.D.8 — Determine the unknown whole number in an addition or subtraction equation.
Who it’s for
This practice is designed for Grade 1 learners building fluency with addition within 20. It is also useful for older learners who want a short review of the number pairs that make 20.
What to know first
- Learners should count reliably to 20, read two-digit numerals, and understand addition as joining parts.
Difficulty and scaffolding
Developing · Abstract stage
Problem representations
- Numerical equation
Three worked examples
- 15 + □ = 20: 15 has 5 ones, and 5 more ones make the next ten, so 15 + 5 = 20.
- 18 + □ = 20: count on from 18 to 19, 20. That is 2 more, so 18 + 2 = 20.
- 11 + □ = 20: 1 and 9 make 10, so 11 + 9 = 20.
Likely misconceptions
- Learners may choose the ones digit of the first number, as in 3 for 13 + □ = 20, or say the first number while counting on and finish one too high. Count on starting with the number after the first, and check that the two numbers add to 20.
Hints and explanations
- 15 + □ = 20: 15 has 5 ones, and 5 more ones make the next ten, so 15 + 5 = 20.
- 18 + □ = 20: count on from 18 to 19, 20. That is 2 more, so 18 + 2 = 20.
- 11 + □ = 20: 1 and 9 make 10, so 11 + 9 = 20.
What to practice next
Next, add a one-digit number to a two-digit number by counting on, with totals up to 98.
Editorial review
Reviewed by Brilliant’s curriculum team. Exercises are generated within defined difficulty ranges and checked against verified answer rules.
Missing addends to make 20 FAQs
What is the goal of missing addends to make 20?
The goal is to find the number that makes 20 with any number from 10 to 19, using numerals alone. A learner should be able to explain the answer by counting on to 20 or by making the next ten.
Should this practice be timed?
No timer is required. First build accurate, understandable strategies. Speed usually improves naturally as learners recognize groups of ten, familiar facts, and repeated patterns.
What should a learner do after a mistake?
Count on from the first number to 20 and keep track of each step, then check that the two numbers add to 20. A short explanation is more valuable than immediately repeating the same guess.
How many problems should be completed at once?
A Quick check has 5 problems and a Practice session has 15. A learner may pause when attention drops. A short, successful session with clear reasoning is more useful than rushing through a longer set.