The purpose of this Number Talk is to elicit the methods students use for subtracting tens from a two-digit number. After students consider and discuss ways to take tens from tens in the first three expressions, students are encouraged to use repeated reasoning to consider subtracting 20 from 35 and adding 1 to compensate (MP8) to find the value of 35−19. These understandings help students develop fluency with operations within 100 and will be helpful later in the lesson when students add and subtract within 100 to solve problems.
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.
Find the value of each expression mentally.
Student Response
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Advancing Student Thinking
Activity Synthesis
Highlight in the first three problems that the tens can be subtracted while the ones are left alone.
Highlight how the result of can be used to calculate . Record this with the equation: .
Activity 1
Standards Alignment
Building On
Addressing
2.OA.A.1
Use addition and subtraction within 100 to solve one- and two-step word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using drawings and equations with a symbol for the unknown number to represent the problem.See Glossary, Table 1.
The purpose of this activity is for students to learn Stage 5 of the Math Stories center. Students pose and solve addition and subtraction story problems about tape diagrams. One student picks a card with a tape diagram without showing their partner. That student poses a story problems that matches the tape diagram. Their partner solves the story problem, then draws a tape diagram and writes an equation to represent the story. Finally, both partners check to see if the tape diagrams are the same.
Launch
Groups of 2
Give each student a recording sheet and a set of cards.
“In this center, you are going to look at diagrams and tell math stories about them.”
“Pick a card without showing your partner and tell your partner a story problem that matches the tape diagram on the card. Then your partner will solve the problem and draw a tape diagram to represent the story. Make sure to also write an equation to represent each story."
“When your partner is finished, check to see if the tape diagrams are the same.”
As needed, demonstrate with a student volunteer.
Activity
15 minutes: partner work time
Monitor for students who tell Put Together/Take Apart or Add To story problems and those who tell Compare story problems with tape diagrams.
None
Student Response
None
Advancing Student Thinking
Activity Synthesis
Invite previously identified students to share.
“What is the same about these story problems? What is different?”
The purpose of this activity is for students to choose from activities that focus on addition and subtraction within 100 or addition and subtraction within 20.
Students choose from stages of these previously introduced centers:
Target Numbers
Capture Squares
Five in a Row
Launch
“Now you will choose from different centers we have learned that focus on adding and subtracting.”
Display the center choices in the student book.
“Think about which activity you would like to do first.”
30 seconds: quiet think time
Activity
Invite students to work at the center of their choice.
8 minutes: center work time
“Choose what you would like to do next.”
8 minutes: center work time
Choose a center.
Target Numbers
Capture Squares
Five in a Row
Student Response
None
Advancing Student Thinking
Activity Synthesis
“Which activity did you choose? What did you like about the activity you chose?”
Lesson Synthesis
“Today we used diagrams to tell our own stories and practiced adding and subtracting.”
“Would you rather create a story problem or solve a story problem? Explain.”
Standards Alignment
Building On
Addressing
2.NBT.B.5
Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.