Lesson 15

The purpose of this Number Talk is to elicit strategies and understandings students have for adding within 1,000. These understandings help students develop fluency and will be helpful when students create their own Number Talk activities.

In the synthesis, discuss what students know about Number Talks and what they need to think about to create one like this example.

• 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.

• “What do you know about Number Talks?” (They always have four expressions and we find their values in our head. Sometimes we can use the same method for each expression. Sometimes we can use the first couple of expressions to help us find the value of other expressions.)
• Consider asking: “What would you have to think about if you were going to design a Number Talk like this one?” (How one expression might relate to the next. What strategies might make it possible for someone to find the value mentally. Whether it's possible to use one expression to find the value of other expressions.)
• Record and display responses for all to see.

The purpose of this activity is for students to reason about subtraction and write another subtraction expression to complete a partially-completed Number Talk activity. If there is time, students can facilitate their Number Talk with another group. Students are given three expressions and prompted to think of ways that they could be evaluated mentally. They then write a fourth expression in which similar reasoning could be used, or by making use of the first three expressions.

• Groups of 2 - 4
• “Here are three expressions like the kinds you might see in a Number Talk.”

• “How would you find the value of each expression, without writing? For each expression, think of at least two ways. Then, share your thinking with your group.”
• Reiterate to students that they are to consider how someone might reason about each difference, rather than only finding the value.
• 4 minutes: independent work time
• 4 minutes: small-group discussion
• “Write one more expression that could go with the other three expressions and would complete a Number Talk activity.”
• 4 minutes: small-group work time
• Monitor for groups who consider the following strategies in writing their expression:
  • adjust the 399 to a different number, but keep the 500
  • leave the 399, but adjust the 500
  • adjust both the 500 and the 399, but create a problem that can be reasoned as in previous expressions
• Identify groups who chose different strategies to share in the synthesis.

• Select groups to share their last expression and their reasoning.
• As each group shares, ask others in the class if they had used the same strategy to write their expression and if they arrived at a similar expression.

The purpose of this activity is for students to reason about addition and apply their reasoning to complete a partially-completed Number Talk activity. This time, students are given addition expressions and are asked to think about how the value of the sums could be found mentally. They then write two new addition expressions to complete the set.

• Groups of 2 - 4
• “Here are two expressions like the kinds you might see in a Number Talk.”

• “How would you find the value of each expression, without writing? For each expression, think of at least two ways. Then, share your thinking with your group.”
• Reiterate to students that they are to consider how someone might reason about each sum, rather than only finding the value.
• 3 minutes: independent work time
• 3 minutes: small-group discussion
• “Write two more expressions that could go with the other two expressions and would complete a Number Talk activity.”
• 5 minutes: small-group work time
• Monitor for groups who consider the following strategies in writing their expression:
  • adjust the 504 to a different number, but keep the 199
  • leave the 504, but adjust the 199
  • adjust the 504 and the 199, but create a problem that can be reasoned as in previous expressions
• Identify groups who chose different strategies to share in the synthesis.

• Select groups to share their two expressions and their reasoning.
• As each group shares, ask others in the class if they had used the same strategy to write their expressions and if they arrived at similar expressions.

The purpose of this activity is for students to write a sequence of three subtraction expressions to complete a Number Talk with only one given expression. As in earlier activities, students consider strategies that they or others might use to mentally subtract numbers within 1,000. 

• Groups of 2 - 4
• “Work with your group to complete a Number Talk with three expressions missing. Write them in the blank lines.”

• 10 minutes: small-group work time
• Monitor for groups who:
  • adjust the 900 or 400 to emphasize a certain subtraction strategy
  • make slight adjustments in each expression but promote the same reasoning strategy throughout 
• Identify groups who choose different strategies to share in the synthesis.

• Select groups to share their completed Number Talk and ask the class how the groups might have chosen the expressions.

The purpose of this activity is for students to design a Number Talk from a blank slate. They can choose to write addition or subtraction expressions, but still consider possible ways that others might find the value of the expressions mentally. If there is time, allow students to facilitate their Number Talk with another group.

• Groups of 1 - 4
• “Work independently or with your group to create a Number Talk activity. You can decide whether to write addition or subtraction expressions.”

• 10 minutes: independent or small-group work time
• Monitor for groups who design their expressions based on certain strategies for subtracting numbers mentally, such as:
  • adding hundreds to hundreds, tens to tens, and ones to ones
  • adding up to subtract
  • relating to benchmarks such as multiples of 100 or 10

• Invite 1–2 groups to share their completed Number Talk or to facilitate it for the class.
• Alternatively, if time permits, allow groups to take turns facilitating their Number Talk with another group.