Lesson 3
Add It, Explain It
Warmup: Number Talk: Add More Tens (10 minutes)
Narrative
The purpose of this Number Talk is to elicit strategies and understandings students have for adding within 100. These understandings help students develop fluency and will be helpful later in this lesson when students add twodigit numbers using methods based on place value.
Launch
 Display one expression.
 “Give me a signal when you have an answer and can explain how you got it.”
 1 minute: quiet think time
Activity
 Record answers and strategy.
 Keep expressions and work displayed.
 Repeat with each expression.
Student Facing
Find the value of each expression mentally.
 \(13 + 21\)
 \(13 + 31\)
 \(23 + 31\)
 \(33 + 41\)
Student Response
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Activity Synthesis
 “How can \(23 + 31\) help you solve \(33 + 41\)?” (The tens are changing again. In \(33 + 41\), you are adding 2 tens total, so the sum of \(23 + 31\) increases by 20. \(54 + 20 = 74\).)
Activity 1: Lin and Han Add (15 minutes)
Narrative
The purpose of this activity is for students to make sense of the first step of addition methods and then finish finding the sum. They write equations to represent their thinking. In the synthesis, students have opportunities to share how they represented a method with equations and connect the representations using their understanding of place value (MP7).
Advances: Representing, Conversing
Required Materials
Materials to Gather
Launch
 Groups of 2
 Give students access to connecting cubes in towers of 10 and singles.
Activity
 Read the task statement.
 7 minutes: partner work time
 Monitor for a student who writes equations to represent how each student can find the value and can explain their thinking.
Student Facing
Lin and Han are both finding the value of \(32+ 54\).
Look at how each student started.
What should they do next to find the value?
Write equations to represent your thinking.
Be ready to explain your thinking in a way that others will understand.

Lin started by putting 3 tens and 5 tens together.
She recorded her first step like this: \(30 + 50 = 80\). 
Han started with 54 and added 3 tens.
He recorded his first step like this: \(54+ 30 = 84\).
Student Response
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Advancing Student Thinking
If students take new steps that do not lead to a value of 86, consider asking:
 “What do you think Lin (or Han) should do next? Why?”
 “What did Lin (or Han) do first? How could you show what they did with connecting cubes?”
 “What do they need to do next? How do you know?”
 “How could you represent the next step with an equation?”
Activity Synthesis
 Invite previously identified students to share their equations for Lin's work and then Han's work.
 “How are these equations the same? How are they different?” (They both got the same sum. They both add tens and ones. The first one breaks apart both numbers and adds tens and tens and ones and ones. The second one starts with 54 and adds the tens then the ones.)
Activity 2: I Heard You Say (20 minutes)
Narrative
The purpose of this activity is for students to find the sum of 2 twodigit numbers and represent their thinking with equations. Students then orally explain their steps to a partner and how the equations they wrote match their steps. Partners listen and then practice restating what they heard their partner say. Restating what a partner has said helps ensure students listen for understanding as their partner explains their thinking. Students have multiple opportunities to explain their thinking and the thinking of a partner and revise their language for clarity (MP3, MP6).
This activity uses MLR8 Discussion Supports. Advances: listening, speaking, conversing.
Supports accessibility for: Organization, Conceptual Processing, Language
Required Materials
Materials to Gather
Launch
 Groups of 2
 Give students access to connecting cubes in towers of 10 and singles.
Activity
 Read the task statement.
 5 minutes: independent work time
 Monitor for students using different methods in order to make partnerships of students who found the value in different ways.
 “Now you are going to explain to your partner how you found the value of the sum. Use your drawings or connecting cubes to explain your steps. Also explain how the equations you wrote match each step you took.”
MLR8 Discussion Supports
 “After your partner explains their method, restate what you heard from your partner.”
 Display the sentence frame: “I heard you say . . .”
 “Ask your partner if you restated their thinking accurately.“
 8 minute: partner discussion
Student Facing
Find the value of \(23 + 74\) using Lin’s or Han’s method.
Write equations to represent your thinking.
\(23 + 74\)
Student Response
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Activity Synthesis
 “What was most difficult about restating your partner’s thinking? What helped you make sense of your partner’s thinking and restate it?”
Lesson Synthesis
Lesson Synthesis
Display:
31 + 48
48 + 30 = 78
78 + 1 = 79
“Today we found the sums of 2 twodigit numbers, explained our thinking, and practiced restating our partner's thinking. Listen as I explain how I found the value of 31 + 48. Then you will have a chance to restate what you heard me say.”
Explain the steps represented by the equations and invite students to share beginning with, “I heard you say...”
Cooldown: Find the Value of $14+53$ (5 minutes)
CoolDown
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Student Section Summary
Student Facing
We added numbers within 100. We learned different methods to add by thinking about tens and ones and record our thinking with equations.
\(23 + 45\)
Add on tens then ones:
\(23 + 40 = 63\)
\(63 + 5 = 68\)
Add tens and tens and ones and ones:
\(20 + 40 = 60\)
\(3 + 5 = 8\)
\(60 + 8 = 68\)