standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Narrative
The purpose of this activity is for students to find the unknown addend in an equation in a way that makes sense to them and compare their methods. In the launch, students are introduced to base-ten blocks and compare them to connecting cubes. During the launch, students should be given time to observe the image and touch the connecting cubes and base-ten blocks.
Students may find the unknown addend using any method that makes sense to them. Monitor and select students with the following methods to share in the synthesis:
count on or count back using connecting cubes or base-ten blocks
use base-ten blocks to show combining or separating tens and ones
use base-ten drawings to show combining or separating tens and ones
Students have the opportunity in the activity and the activity synthesis to consider the available tools and make a choice that best helps them find the unknown addend (MP5). To support student reflection on the utility of each tool, provide each group with towers of ten connecting cubes, but not enough to represent the numbers in the equation without needing to create new towers of ten.
MLR8 Discussion Supports.
When groups compare methods, invite students to take turns sharing their responses. Ask students to restate what they heard using precise mathematical language and their own words. Display the sentence frame: “I heard you say . . . .” Original speakers can agree or clarify for their partner.
Advances: Listening, Speaking
Required Materials
Materials to Gather
Base-ten blocks
Connecting cubes
Required Preparation
Each group of 2 needs 90
–100 connecting cubes, but
no more than 3
–
5 towers of 10 cubes should be included in their collection. Break apart any extra towers for this activity.
Launch
Groups of 2
Give each group towers of 10, single connecting cubes, and base-ten blocks.
Display the image.
“Each group has some connecting cubes and some base-ten blocks.”
“What is the same and what is different between these tools?” (They both are cubes or towers of cubes. The connecting cubes are in towers of 10, single cubes, and some are in towers of different sizes. The blocks are only in tens and ones. The blocks in tens do not come apart.)
1 minute: quiet think time
1 minute: partner discussion
Share responses.
Activity
“Work together to find the number that makes the equation true. You can use the connecting cubes, base-ten blocks, or other representations to help find the number or show your thinking. Be prepared to explain your thinking.”
6 minutes: partner work time
As students work, consider asking:
“Why did you choose this tool?”
“How did you find the number that makes the equation true?”
“What is another way you could use this tool to find the unknown number?”
“Now compare your method with another group. How are your methods the same? How are they different?”
2 minutes: group discussion
Student Facing
[IMG]
What is the same and what is different between these tools?
Find the number that makes the equation true. Show your thinking using the cubes, blocks, or drawings.
\(41 + \underline{\phantom{\hspace{1.05cm}}}=84\)
Student Response
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Activity Synthesis
Invite previously identified groups to share their methods in the given order.
Consider asking:
“Why did you choose this tool to help you find the number?”
“How are the methods the same? How are they different?” (Some methods are the same, they just used different tools to show it. Some methods used the same tool, but are different because one group added tens and ones to find the unknown number, but another group took away tens and ones to find the unknown number.)


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