standard_description: Fluently add and subtract within 20 using mental strategies. (See standard 1.OA.6 for a list of mental strategies.) By end of Grade 2, know from memory all sums of two one-digit numbers.
problem_activity: Narrative
The purpose of this activity is for students to add or subtract to find unknown addends within 20. Students will find unknown addends when the total is 20. The known addends may encourage students to think about ways to make a ten and to use the sums and differences that they know (MP7).
MLR7 Compare and Connect.
Synthesis: After all strategies have been presented, lead a discussion comparing, contrasting, and connecting the different approaches. Ask, “Why did the different approaches lead to the same outcome? What did the methods have in common? How were they different?”
Advances: Representing, Conversing
Engagement: Develop Effort and Persistence.
Students may benefit from feedback that emphasizes effort and time on task. For example, give feedback on the partner relationship and how students are able to work together to complete a task. Encourage both partners to share ideas and speak up to agree or disagree.
Supports accessibility for: Social-Emotional Functioning, Attention
Required Materials
Materials to Copy
Number Cards 0-19
Required Preparation
Create a set of cards for each group of 2.
Launch
Groups of 2
Give each group a set of cards.
Activity
“Let’s play another game to find unknown addends.”
Select a volunteer to be your partner.
“Partner A will draw a card and hold it on their forehead without looking.”
Prompt the volunteer to demonstrate.
“Partner B names a number that you can add to the number on the card to get to 20.”
Think aloud finding and naming the number.
“Partner A then names the number they are holding on their head.”
Prompt the volunteer to find the number.
“Check your work and write an equation to represent how you found an unknown addend in the turn. Share your equation with your partner. Switch roles and play again.”
12 minutes: partner work time
Monitor for students who:
use place value reasoning or look to make 10
use known addition or subtraction facts
make equivalent, but easier sums or differences
Student Facing
Directions:
Partner A holds a card on their forehead without looking.
Partner B names the number that you can add to the number on the card to get 20.
Partner A says the number on their card.
Record an equation to represent how you found the unknown addend.
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Student Response
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Activity Synthesis
“Mai thought of trying to make 2 tens. What number needs to be added to 12 to make 2 tens?”
“What other methods did you use to find the unknown addend?”
Invite previously identified students to share methods.
“Pull out the number cards for 0–9, as you will use them in the next activity.”


aligns: yes