Lesson 13
Analyze Addition Mistakes
Warmup: Estimation Exploration: Many Places (10 minutes)
Narrative
The purpose of this Estimation Exploration is for students to estimate a sum of two decimals. The numbers are complex to encourage students to make an estimate which means identifying that the leading digits, rather than the decimal places, are the most important for making a good estimate.
In this lesson, students start to work with sums of larger numbers. Making a mental estimate before calculating is a valuable skill to help confirm the reasonableness of a solution.
Launch
 Groups of 2
 Display the expression.
 “What is an estimate that’s too high?” “Too low?” “About right?”
 1 minute: quiet think time
Activity
 “Discuss your thinking with your partner.”
 1 minute: partner discussion
 Record responses.
Student Facing
\(1,\!987.89 + 658.54\)
Record an estimate that is:
too low  about right  too high 

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Student Response
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Activity Synthesis
 “Which digits in the numbers were most important for making your estimate?” (The 1 and first 9 from from the first number and the 6 and 5 from the second number.)
Activity 1: Compare Calculations (20 minutes)
Narrative
The purpose of this activity is for students to analyze a common error when using the standard algorithm to add decimals. The standard addition algorithm requires students to add digits with the same place value. In the given example the two numbers are “right aligned” as when adding whole numbers but this leads to the error of adding tenths from one number to hundredths of the other number as if they had the same place value. When students analyze the incorrect calculation, explain why it is incorrect, and correct it they critique the reasoning of others (MP3).
This activity uses MLR3 Clarify, Critique, and Correct. Advances: Reading, Writing, Representing.
Launch
 Groups of 2
 “Solve the first problem on your own.”
 2–3 minutes: independent work time
 “Now, work on the second problem on your own for a few minutes, and then talk to your partner about it.”
Activity
 1–2 minutes: independent work time
 6–8 minutes: partner work time
 Monitor for students who:
 use estimation to reason (For example, they notice that adding almost 70 to 620 must be a lot greater than 628, as shown in Elena’s calculation.)
 notice the misalignment of the decimal places being added in Elena’s calculation
Student Facing
 Find the value of \(621.45 + 72.3\). Explain or show your reasoning.
 Elena and Andre found the value of \(621.45 + 72.3\). Who do you agree with? Explain or show your reasoning.
Student Response
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Advancing Student Thinking
If students do not have a strategy to solve the first problem, ask “What strategy would you use to find the value of 621 + 72?”
Activity Synthesis
 Display Elena’s partially correct answer and explanation.
 Read the explanation aloud.
 “What do you think Elena means? Is anything unclear?” (She says the answer will be more than 621, but she doesn’t say how much more. The answer will be a lot more than 628.68.)
 “With your partner, work together to write a revised solution and explanation.”
 (Optional) Display and review the following criteria:
 Explanation of mistakes
 Specific words and phrases
 Decimal point
 Place value
 Labeled diagram
 Correct solution
 3–5 minutes: partner work time
 Select 1–2 groups to share their revised explanation with the class.
 “What is the same and different about the revised solutions and explanations?”
 Invite previously selected students to share.
 Display Andre’s solution.
 “How does Andre know that he needs to add a little more than 3 to 690?” (He hasn’t added the ones, tenths, or hundredths places yet and \(1.45 + 2.30\) is a little more than 3.)
 “Why do you think Andre uses 72.30 instead of 72.3?” (It helps him keep the place values and decimals in line so he doesn’t mix up the places when he is adding. 72.30 is equivalent to 72.3.)
Activity 2: Same Digits, Different Sums (15 minutes)
Narrative
The purpose of this activity is for students to find the value of various decimal sums with no scaffold. Most of the numbers do not have the same number of decimal digits so students need to add carefully if they make vertical calculations, making sure to align place values correctly (MP6).
Supports accessibility for: Conceptual Processing, Attention
Launch
 Groups of 2
Activity
 5 minutes: independent work time
 2 minutes: partner discussion
 Monitor for students who
 use the standard algorithm
 use place value understanding but do not organize their calculations vertically
Student Facing
Find the value of each expression. Explain or show your reasoning.

\(2.63 + 7.74\)

\(26.3 + 774\)

\(46.3 + 31.42\)

\(463 + 3.14\)
Student Response
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Advancing Student Thinking
If students do not find the correct value of a sum, ask, “Which 2 whole numbers will the value of the sum be between?”
Activity Synthesis
 Invite previously selected students to share.
 Display: \(463 +3.14\)
 “How do you add these using the standard algorithm?” (I made sure that the 3 in 463 lines up with the 3 in 3.14 so I am adding numbers with the same place value. I added 0’s in the tenths and hundredths places of 463 to help add the numbers.)
 “Did anyone use a different strategy to add these numbers?” (I knew that 463 and 3 is 466 and then I also have 14 hundredths so it's 466.14.)
Lesson Synthesis
Lesson Synthesis
“Today we analyzed errors in strategies for adding decimals.”
“Describe what is the most important thing to remember when we add decimal numbers.” (Pay attention to the place values of the digits that I add together.)
“What are some mistakes someone can make when finding the sum?” (Adding different place value positions. Making an error in writing the expression vertically by not lining up the decimal. Forgetting to write down newly composed units.)
Cooldown: What is the Error? (5 minutes)
CoolDown
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