Lesson 12
Mentally Add and Subtract Tens (optional)
Warmup: Number Talk: Add and Subtract 10 (10 minutes)
Narrative
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
 \(3 + 10\)
 \(10 + 5\)
 \(13  10\)
 \(15  10\)
Student Response
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Activity Synthesis
 “Did anyone approach the problem in a different way?”
Activity 1: Introduce Write Numbers, Numbers to 99 by 10 (20 minutes)
Narrative
The purpose of this activity is for students to learn stage 2 of the Write Numbers center. Like the last stage of this center, students take turns writing the next one, two, or three numbers. The player who writes the last number on the board wins. Students may choose to count forward or backward. In this stage, students count by 10 starting at numbers other than 10.
Advances: Speaking
Required Materials
Materials to Gather
Materials to Copy
 Write the Number Stage 2 Gameboard
Required Preparation
 Put each gameboard in a sheet protector.
Launch
 Groups of 2
 Give each group a gameboard and a dry erase marker.
 “We are going to learn a new way to do the Write Numbers center.”
 Display the gameboard.
 “You and your partner will practice writing numbers. Just like the last time we played this game, you will fill in the number path on the gameboard. You can decide to start with the smaller number and count forward, or start with the larger number and count backward. On each turn, you can decide whether you would like to write one, two, or three numbers on the gameboard. The person who writes the last number on the board is the winner.”
 “Today you will not write every number in the sequence. You will count by ten and write each number you say.”
 Demonstrate playing one round with the students.
 “Now you will play with your partner.”
Activity
 10 minutes: partner work time
Activity Synthesis
 “What do you notice about each number on the gameboard?” (They all have the same amount of ones. The tens go up by one in each number.)
Activity 2: Add and Subtract 10 (20 minutes)
Narrative
The purpose of this activity is for students to practice adding and subtracting 10 mentally from any twodigit number. The problems are organized into two sets. In the first set, students add and subtract 10 from the same number. Students may notice a pattern and generalize that when you add or subtract 10, the number of tens increases by one or decreases by one and that the ones place does not change (MP7). In the second set, students work with a range of numbers including adding or subtracting multiple tens to develop fluency.
Supports accessibility for: Conceptual Processing, Memory
Launch
 Groups of 2
 “Today you will add or subtract 10 from twodigit numbers and find the value that makes each equation true. First, you will complete some equations. When you finish all of the equations, you will talk with your partner about the patterns you noticed, then each partner will write about it.”
 “Then, you will complete some more equations, but after each set of equations you will talk with your partner about what you notice then write about it.”
Activity
 15 minutes: partner work time
Student Facing
 Find the number that makes each equation true.
Then tell what you notice.
\(67 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(67  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\) 
\(39 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(39  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\) 
\(52 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(52  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\) 
\(75 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(75  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\) 
Talk to your partner. What patterns do you notice?
I notice that when I add 10,
I notice that when I subtract 10,

\(67 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)

Find the number that makes each equation true.
After each set of equations, tell what pattern you notice.
\(67 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(67 + 10 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(67 + 10 + 10 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
I notice that 
\(99  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(99  10  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(99  10  10  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
I notice that 
\(45 + 10 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(45  10  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
\(45 + 10  10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
I notice that

\(67 + 10 = \boxed{\phantom{\frac{aaai}{aaai}}}\)
Student Response
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Advancing Student Thinking
If students find the values of the expressions by counting on or counting back by one, consider asking:
 “How did you find the number to make this equation true?”
 “How could you use what you know about tens and ones to find the number that makes the equation true?”
Activity Synthesis
 “What patterns did you notice as you added or subtracted 10?” (Only the tens place changes. In some of the equations I was adding 2 tens or 3 tens so it was like adding tens from before. I can skip count to add or subtract tens quickly.)
Lesson Synthesis
Lesson Synthesis
Display 67 and 77.
“Today we added and subtracted 10 from other numbers in our head. What statements can you make about these two numbers?” (77 is more than 67. It is 10 more than 67. 67 is 10 less than 77. They both have 7 ones.)
If needed, ask, “How much more is 77 than 67?”
Display 82 and 62.
“What statements can you make about these two numbers?” (82 is more than 62. It is 20 more than 62. 62 is less than 82. It is 20 less than 82.)
Cooldown: Unit 4, Section B Checkpoint (0 minutes)
CoolDown
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