Lesson 27

The purpose of this Number Talk is to elicit strategies and understandings students have for subtracting a teen number. The expressions are written to encourage students to notice a teen number can be broken into a ten and some ones and subtracted in parts.

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

• Record answers and strategy.
• Keep expressions and work displayed.
• Repeat with each expression.

• Display \(20 - 10 - 1\).
• “How much did we subtract from 20 in this expression? What other expression can we write to show this?” (\(20 - 11\))

The purpose of this activity is for students to learn stage 2 of the How Close center. Students pick four number cards and then choose two or three of the numbers to subtract from 20 to get as close as possible to zero.

• Groups of 2
• Give each group a set of number cards, two recording sheets, and access to double 10-frames and connecting cubes or two-color counters.
• “We are going to learn a new way to play the How Close center. Let's play one round together.”
• Display four cards.
• “Each player starts with four cards. Choose numbers to subtract from 20 in order to get as close as possible to zero. You can choose two or three numbers to subtract.”
• “Which of these numbers should I subtract from 20 to get close to zero?”
• Share responses.
• “After you have chosen the numbers you will subtract, complete the equation on your recording sheet.”
• Demonstrate writing the equation.
• “Compare your work with your partner's. The player who gets closer to zero gets one point. The person who gets more points wins.”

• 10 minutes: partner work time

• Display number cards 6, 4, 9, 8
• “Which cards would you use and why?” (9 and 8 are the biggest numbers so that would get me close to 0. 6 and 4 make 10 and if we include 9 that would get us closer to 0.)

The purpose of this activity is for students to learn stage 2 of the Compare center. Students choose a card with either an addition or subtraction expression and compare the values. After learning this new stage of the Compare center, students then choose from other activities focused on addition and subtraction.

Students choose from any stage of previously introduced centers.

• Compare
• How Close?
• Five in a Row

• Create a set of Compare Stage 2 Subtraction Cards for each group of 2.
• Each group of 2 needs a set of Compare Stage 2 Addition Cards from a previous lesson.
• Gather materials from previous centers:
  • Compare, Stage 1
  • How Close? Stages 1 and 2
  • Five in a Row, Stages 1-3

• Groups of 2
• Give each group a set of addition and subtraction cards and access to double 10-frames and connecting cubes or two-color counters.
• “We are going to learn a new way to play the Compare center. You will still find the value of the expression on your card and compare values with your partner. The partner whose card has the greater value takes both cards. The game is over when each partner runs out of cards to flip over. The partner with the most cards wins.”

• 8 minutes: partner work time
• “Now you will choose from centers we have already learned. One of the choices is to continue with Compare.”
• Display the center choices in the student book.
• “Think about what you would like to do.”
• 30 seconds: quiet think time
• Invite students to work at the center of their choice.
• 10 minutes: center work time

• Display \(9 + 6\) and \(19 - 6\).
• “Which card has the greater value? How do you know?”