# Lesson 12

Number Talk

## Warm-up: Number Talk: A Whole Number and a Fraction (10 minutes)

### Narrative

This Number Talk encourages students to rely on properties of operations and what they know about multiplication of a fraction and a whole number to mentally solve problems. The understandings elicited here will be helpful later in the lesson when students complete or create Number Talk activities.

### Launch

• Display one expression.
• “Give me a signal when you have an answer and can explain how you got it.”

### Activity

• 1 minute: quiet think time
• Keep expressions and work displayed.
• Repeat with each expression.

### Student Facing

Find the value of each expression mentally.

• $$6 \times \frac{1}{4}$$
• $$6 \times \frac{3}{4}$$
• $$18 \times \frac{3}{4}$$
• $$180 \times \frac{3}{4}$$

### Activity Synthesis

• “Today we are going to write our own Number Talks. Imagine the writer of this Number Talk started with $$6 \times \frac{1}{4}$$. What did you notice about how each expression changed? What do you think the writer was trying to get you to notice?” (Each expression had a factor that changed by multiplying by 3 for the first two expressions, then by 10 for the last expression. I think they were getting us to notice how you can look for ways to make each expression easier to solve by using products we've already found.)
• As needed, “How is each expression like the expression before it? How can you use that change to find each new value?” (Each expression has one factor that changes. If you think about how many times greater the factor is than the factor before it, you can use that to find the product.)

## Activity 1: Related Numbers, Related Expressions (20 minutes)

### Narrative

A Number Talk activity encourages students to look for structure and relationships that can help them reason about expressions. This activity puts students in the mindset of a Number Talk writer. It prompts students to anticipate some ways that others might decompose, rearrange, and regroup numbers, or to otherwise make use of structure to find the value of expressions. The work here will be helpful as students consider the numbers and numerical relationships to use as they write their own expressions later.

Action and Expression: Develop Expression and Communication. Provide students with alternatives to writing on paper: students can explain their strategies orally or using manipulatives, such as base-ten blocks.
Supports accessibility for: Language, Organization, Memory

### Launch

• Groups of 2
• “Find at least two ways to mentally find the value of each expression, without writing.”

### Activity

• 7–8 minutes: independent work time
• 2–3 minutes: partner discussion

### Student Facing

1. Here are two addition expressions. Think of at least two different ways to find the value of each sum mentally.

1. $$15 + 29$$
2. $$30 + 58$$
2. Here are three subtraction expressions. Think of at least two different ways to find the value of each difference mentally.

1. $$91 - 11$$
2. $$91 - 16$$
3. $$391 - 86$$
3. Can you write a fourth subtraction expression that uses the same strategy you used to find the value of the other differences?

### Activity Synthesis

• Invite students to share their mental computation strategies. Collect as many strategies as time permits and record them.
• If no students mentioned making use of one or more preceding expressions in their mental computation, ask them how it could be done. (See student response for examples.)
• Select a couple of students to share their new expressions. Ask the class to find the value of the expressions mentally.

## Activity 2: Add One New Expression, Then Two (15 minutes)

### Narrative

In this activity, students use their understanding of place value and knowledge of operations on numbers to write new multiplication and division expressions. Students study the given expressions and their relationships and consider how they could find the value of each expression mentally. Next, they propose new expressions to complete each Number Talk activity.

MLR7 Compare and Connect. Synthesis: After all strategies have been presented, lead a discussion comparing, contrasting, and connecting the different approaches. Ask, “Why did the different approaches lead to the same (or different) outcome(s)?” or “What did the approaches have in common? How were they different?”

### Launch

• Groups of 2
• “You’ll see two incomplete Number Talk sets, each with one or more missing expressions. Find the value of each expression mentally and think about their relationship.”

### Activity

• “Write one or more new expressions to complete the sets. Be prepared to explain your reasoning for the expression.”
• 7–8 minutes: group work time
• Monitor for students who:
• adjust the dividend but keep the divisor
• adjust the divisor but keep the dividend
• use place value to multiply or divide
• use multiplicative relationships such as halving and doubling and reason how it impacts the quotient

### Student Facing

1. Here are three division expressions. Find the value of each quotient mentally and think about how they might be related.

• $$35 \div 5$$

• $$70 \div 5$$

• $$210 \div 5$$

• ___________________

Write a new division expression whose value can be found more easily after working through the first three.

2. Here are two multiplication expressions. Analyze them and think about how they might be related.

• $$21 \times 7$$

• $$42 \times 7$$

• ___________________

• ___________________

Write two new expressions. Be prepared to explain your reasoning for each expression.

### Activity Synthesis

• Select students who reasoned differently to share their expressions. Record each set.
• Ask the class to determine the reasoning behind the new expression(s) and ask the writers to respond to the feedback.
• “How might the second expression help someone find the value of the third expression?”
• “How might the third expression help someone find the value of the fourth?”

## Activity 3: Add Three New Expressions [OPTIONAL] (30 minutes)

### Narrative

In this optional activity students are given four expressions—three expressions involve operations of two whole numbers and one involves multiplication of a whole number and a fraction. Students choose one expression and write three new ones to create a Number Talk activity.

Students have considerably more freedom to decide the direction of subsequent expressions, but should be just as prepared to explain the rationale behind their expressions. Expect the increased openness to be a greater cognitive lift for students.

For the activity synthesis, ask each group of 2 to test their set by presenting it to another group and attending to how others reason about their expressions.

• Groups of 2

### Activity

• “Choose a starting expression. Then, work with your group to write three new expressions to make a complete Number Talk activity. Be prepared to explain the reasoning for each expression.”
• “Each new expression may be more challenging to evaluate on its own, but easier to evaluate after working through the ones before it.”
• 15 minutes: group work time

### Student Facing

Here are four expressions you could use to start a Number Talk activity.

$$75 + 30$$

$$160 - 51$$

$$24 \div 8$$

$$3 \times \frac{1}{6}$$

1. Choose one starting expression. Think of at least two different ways to find its value mentally.
2. Write three equations to create a Number Talk activity. Be prepared to explain your reasoning for writing each expression.

___________________ (starting expression)

___________________

___________________

___________________

3. Create an answer key for your Number Talk. Include at least one way to find the value of each expression mentally.

### Activity Synthesis

• “Work with another group and take turns presenting your Number Talk activity.”
• “Show one expression at a time. Move on only after an answer and an explanation are given. Record their reasoning.”
• After students have a chance to test their Number Talk, discuss questions such as:
• “Did others reason in a way you anticipated?”
• “Were there expressions that you’d revise?”

## Lesson Synthesis

### Lesson Synthesis

“Today (or the past couple of days) you’ve used your mathematical understanding to write expressions for Number Talk activities. You’ve also created an original set.“

“What were important things you considered as you wrote expressions for a Number Talk? Why were these things important?”

“What were some challenges in creating a brand new Number Talk, or in completing a partial set?”