Lesson 11
Add Tens to Twodigit Numbers
Warmup: True or False: Tens and Ones (10 minutes)
Narrative
The purpose of this True or False is to elicit insights students have about adding twodigit numbers using place value understanding. This will be helpful later when students add a twodigit number and a multiple of 10.
Launch
 Display one statement.
 “Give me a signal when you know whether the statement is true and can explain how you know.”
 1 minute: quiet think time
 Share and record answers and strategy.
 Repeat with each equation.
Student Facing
Decide if each statement is true or false.
Be prepared to explain your reasoning.
 \(80 + 5 = 5 + 80\)
 \(70 + 1 = 80 + 1\)
 \(20 + 6 = 6 + 30\)
Student Response
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Activity Synthesis
 “How can you use what you know about tens and ones to reason about the equations?” (Two numbers are not equal if the number of ones is the same but the tens is different.)
Activity 1: Add Twodigit Numbers and Tens (20 minutes)
Narrative
The purpose of this activity is for students to add multiples of 10 and twodigit numbers in a way that makes sense to them. Students may use connecting cubes in towers of 10 and singles. They represent their thinking using drawings, numbers, or words. Some students may make baseten drawings to show the addition, while others may find the sum mentally, applying what they have learned about place value in previous lessons. Some students may use both methods, depending on the numbers in the problem.
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.
 10 minutes: partner work time
 Monitor for students who find the sum of 30 + 65 by:
 drawing and counting by ten, then one
 count on from 65 by ten: 75, 85, 95
 combine 3 tens and 6 tens, then add 5 ones
Student Facing
Show your thinking using drawings, numbers, or words.
 \(37 + 20 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
 \(60 + 23 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
 \(48 + 50 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
 \(\boxed{\phantom{\frac{aaai}{aaai}}} = 54 + 20\)
 \(30 + 65 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
Student Response
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Activity Synthesis
 Invite previously identified students to share.
 “How are these methods the same? How are they different?” (They all added tens to tens. Some people started with the first number and other people started with the second number.)
Activity 2: The Missing Digit (15 minutes)
Narrative
The purpose of this activity is for students to use their understanding of place value to make an equation true when a digit in a twodigit number is “missing.” Students consider the values of the digits in order to justify their thinking. Students may use connecting cubes in towers of 10 and singles, create drawings, or write about their reasoning.
Advances: Reading, Representing
Supports accessibility for: Organization, Attention
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.
 10 minutes: partner work time
Student Facing

This equation is true.
What digit is under the smudge?
Show your thinking using drawings, numbers, or words.  This equation is not true.
Show why it is not true using drawings, numbers, or words.
Student Response
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Activity Synthesis
 Invite students to share how they determined the missing digit in the first equation.
 Invite students to share how they know the second equation cannot be true.
Lesson Synthesis
Lesson Synthesis
Display \(34 + 40\).
“Today we added tens to twodigit numbers. What do we know about adding these numbers?” (It’s like counting on by tens. You can think of it as 34 and 4 more tens. You can just add the number of tens in each number. For example, 3 tens + 4 tens is 7 tens.)
Cooldown: Add Tens (5 minutes)
CoolDown
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Student Section Summary
Student Facing
We learned that twodigit numbers are made up of tens and ones.
We represented twodigit numbers in many different ways.
6 tens and 5 ones
\(60+5\)
65
We added twodigit numbers by thinking about counting on by 10 or adding more tens.
\(42 + 50 = \boxed{92}\)