Lesson 5

The purpose of this Number Talk is to elicit strategies and understandings students have for subtracting 1 or 2 from a given number. Students may use the structure of the count sequence and count back to determine the difference (MP7).

• Display one expression.
• “Give me a signal when you have an answer and can explain how you got it.”
• 1 minute: quiet think time

• Share and record responses.
• Keep problems and work displayed.
• Repeat with each expression.

• “How are problems 3 and 4 the same?  How are they different?” (They both start with 8. They are both subtraction. One subtracts 1 and the other subtracts 2. One is 7 and the other is 6.)

The purpose of this activity is for students to learn stage 2 in the center Check it Off. In this stage students subtract within 10 in any way that makes sense to them. Students may count all and remove some before counting again, count back, or use a known fact. Students are introduced to the term difference. They write subtraction expressions to represent differences. If students write an expression with the minuend and subtrahend in reversed order, for example \(4 - 7\) instead of \(7 - 4\), ask them to represent the expression with cubes or two-color counters. Students will see that if they have four cubes, they are not able to take away seven. Allow students to think about how they can write an expression that represents beginning with one of the numbers and taking away the other to find the difference. Avoid telling students that the bigger number is always first in a subtraction expression since this will not remain true when students begin working with negative numbers. 

• Each group of 2 needs a set of Number Cards 0-10.

• Groups of 2
• Give each group a set of number cards, two recording sheets, and access to two-color counters and 10-frames.
• “We are going to learn a new way to play Check it Off. This time, instead of adding to find each number on the recording sheet, you will subtract.”
• Demonstrate choosing two number cards.
• “Now I find the difference between the two numbers. The difference is the result when one number is subtracted from another. What is the difference between these two numbers? How do you know?”
• Demonstrate checking off the number on the recording sheet.
• “After you check off the number, write a subtraction expression to show the difference.”
• Demonstrate writing the subtraction expression.
• “Continue taking turns with your partner. The person who checks off the most numbers wins.”

• 15 minutes: partner work time
• As students work, consider asking:
  • “How did you find the difference?”
  • “Which expression did you write?”
• Monitor for students who count out counters, remove some and count again or count back to find the difference.

• “I have the numbers 7 and 3. How can I find the difference?”
• Invite previously identified students to share.
• “What subtraction expression can I write to show the difference?”

The purpose of this activity is for students to choose from activities that focus on counting up to 20 objects or adding and subtracting within 10. Students choose from any stage of previously introduced centers and are encouraged to choose the center that will be most helpful for them at this time. 

• Counting Collections
• Number Race
• Check it Off
• Five in a Row: Addition and Subtraction

• Gather materials from previous centers:
  • Counting Collections, Stage 1
  • Number Race, Stage 3
  • Check it Off, Stages 1 and 2
  • Five in a Row: Addition and Subtraction, Stages 1 and 2

• Groups of 2
• “Now you are going to choose from centers we have already learned.”
• Display the center choices in the student book.
• “Think about what you would like to do first.”
• 30 seconds: quiet think time

• 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

• “We have been playing lots of games with addition and subtraction. What is something you have gotten better at from playing these games?”