# Lesson 19

Use Properties to Multiply Decimals

## Warm-up: Number Talk: Many Hundredths (10 minutes)

### Narrative

The purpose of this Number Talk is for students to demonstrate strategies and understandings they have for place value relationships and the properties of operations as they find the value of different products (MP7). The products all have the same value, 6, and also all have a decimal factor of 0.1 or 0.01. The whole number factors are organized differently and this encourages students to think flexibly about how to find products of a whole number and a decimal.

### 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

• Keep expressions and work displayed.
• Repeat with each expression.

### Student Facing

Find the value of each expression mentally.

• $$40 \times 2 \times 0.1$$
• $$20 \times 0.1 \times 4$$
• $$0.1\times 80$$
• $$0.01 \times 20 \times 40$$

### Student Response

For access, consult one of our IM Certified Partners.

### Activity Synthesis

• “How is the last expression different from the others?” (It has a hundredth in the product instead of a tenth.)
• “How did you find the value of this expression?” (I knew $$40 \times 20 = 800$$ and then 800 hundredths is 8.)

## Activity 1: Card Sort: Decimal Multiplication Card Sort (20 minutes)

### Narrative

The purpose of this activity is for students to use properties of operations to develop strategies for multiplying decimals (MP7). They first sort expressions into groups which can be used to find the value of a given decimal product. Then they choose one of the expressions to find the value. Many of the expressions use whole number products and the associative property which students have seen in previous lessons. Some of the expressions use subtraction and a compensation strategy. This strategy is new for decimals but will be familiar to students for whole number products.

### Required Materials

Materials to Copy

• Decimal Multiplication Expression Card Sort

### Launch

• Groups of 2
• Distribute one set of pre-cut cards to each group of students.

### Activity

• “In this activity, you will sort some expressions into categories of your choosing. When you sort the expressions, work with your partner to come up with categories.”
• 3 minutes: partner work time
• “Each expression matches one of the expressions marked A, B, or C.  Work with your partner to match the cards. Justify your choices.”
• 8 minutes: partner work time
MLR2 Collect and Display
• Circulate, listen for and collect the language students use to explain how they know expressions are equal. Listen for words such as groups of, the same as, and break apart. For example, students may say:
• 4 groups of 39 tenths is the same as 4 groups of 3 and 9 tenths.
• 4 times 3 tenths plus 4 times 5 hundredths is the same as 4 times 35 hundredths.
• 2 groups of 2 groups of three and 5 tenths is the same as 4 groups of 3 and 5 tenths.
• Record students’ words and phrases on a visual display and update it throughout the lesson.

### Student Facing

1. Your teacher will give you a set of cards that show multiplication expressions.

1. Sort the cards into 2 categories of your choosing. Then, sort the cards into 2 categories in a different way. Be prepared to explain the meaning of your new categories. (Pause for teacher directions.)
2. There are three expressions labeled A, B, and C. The value of each of the other expressions is equal to one of these. Match the expressions. Be prepared to explain your reasoning.
2. Choose one expression from each group to find the value of the expressions on cards A, B, and C.
3. Write at least one more expression that is equal to each of the expressions on cards A, B, and C.

### Student Response

For access, consult one of our IM Certified Partners.

### Activity Synthesis

• Invite students to share the matches they made and explain how they know those cards go together.
• Refer to the language on the display as students describe their justification for a match, giving them opportunities to describe the relationship more precisely.
• “Are there any other words or phrases that are important to include on our display?”
• As students share responses, update the display, by adding (or replacing) language, diagrams, or annotations.
• Remind students to borrow language from the display as needed.
• Invite selected students to share their strategies for calculating $$4 \times 3.9$$.
• “Why is $$(4 \times 4) - (4 \times 0.1)$$ a useful expression for finding the value of $$4 \times 3.9$$?” (I can find those products in my head.)
• “Why is $$(4 \times 39) \times 0.1$$ a useful expression for finding the value of $$4 \times 3.9$$?” (I know how to find products of whole numbers like $$4 \times 39$$ and then multiplying by 0.1 just changes place values.)

## Activity 2: Choose a Strategy (15 minutes)

### Narrative

The purpose of this activity is for students to find products of a whole number and a decimal where the decimal has more than one place value, either a whole number and some tenths or some tenths and some hundredths. Monitor for these strategies which students saw in the previous activity

• multiplying whole numbers and then multiplying the result by 0.1 or 0.01
• using the distributive property and multiplying by place value
• using the distributive property and compensation
Engagement: Provide Access by Recruiting Interest. Synthesis: Revisit math community norms to prepare students for the whole-class discussion.
Supports accessibility for: Language, Social-Emotional Functioning

• Groups of 2

### Activity

• 1–2 minutes: quiet think time
• 6–8 minutes: partner work time
• Monitor for students who find the products using
• whole number products and place value understanding
• the distributive property

### Student Facing

Find the value of each expression. Explain or show your reasoning.

1. $$6 \times 0.12$$
2. $$4 \times 1.4$$
3. $$5 \times 3.9$$
4. $$25 \times 0.41$$

### Student Response

For access, consult one of our IM Certified Partners.

If students need support with the placement of the decimal in the product, ask them to use the language on the display from activity 1 to describe each of the expressions.

### Activity Synthesis

• Invite selected students to share their reasoning for the value of $$4 \times 1.4$$.
• Display expression: $$(4 \times 14) \times 0.1$$
• “Why is this expression helpful to find the value of $$4 \times 1.4$$?” (I know $$4 \times 14$$ is 56. I can do that with whole number multiplication. Then it’s that many tenths so 5.6.)
• Display expression: $$(4 \times 1) + (4 \times 0.4)$$
• “Why is this expression helpful to find the value of $$4 \times 1.4$$?” (It breaks it up by place value. I first find 4 ones and then 4 times 4 tenths. That’s 4 and 16 tenths or 5 and 6 tenths so it’s 5.6.)
• “How did you choose a strategy for each problem?” (I like to multiply whole numbers so I always thought of products of whole numbers and then took that many tenths or hundredths. I noticed 3.9 is really close to 4 and I know $$5 \times 4$$ so I started there and figured out what I needed to subtract.)

## Activity 3: More Multiplication Problems (Optional) [OPTIONAL] (10 minutes)

### Narrative

The purpose of this optional activity is to find more complex products of a whole number and a decimal using any strategy. For the more complex numbers, the strategies that students have seen all apply but the most reliable one is to find a product of whole numbers and then identify the number of tenths or hundredths that is. The distributive property is still an effective tool but a product of 2 two-digit numbers gives 4 single digit products. The problems are scaffolded so that students can use their answers for the first two problems to find the answer to the third.

### Activity

• 5 minutes: independent work time
• Monitor for students who use the first two calculations for the third and who use place value understanding for the last calculation.

### Student Facing

Find the value of each expression.

1. $$35 \times 0.08$$
2. $$35 \times 0.7$$
3. $$35 \times 0.78$$
4. $$42 \times 0.66$$

### Student Response

For access, consult one of our IM Certified Partners.

### Activity Synthesis

• Invite students to share their calculations.
• “How did you use the values of $$35 \times 0.08$$ and $$35 \times 0.7$$ to find the value of $$35 \times 0.78$$?” (I added them.)
• Display equation: $$35 \times 0.78 = (35 \times 0.7) + (35 \times 0.08)$$
• “How do you know this equation is true?” (It’s the distributive property.)
• “How did you calculate $$42 \times 0.66$$?” (I found $$42 \times 66$$ and then knew I had that many hundredths.)

## Lesson Synthesis

### Lesson Synthesis

“Today we used different strategies to multiply whole numbers by decimals.”

“What are some different strategies we used to multiply whole numbers by decimals?” (We multiplied whole numbers by 0.1 or 0.01. We broke the decimal apart, multiplied the whole number by the different parts, and then added or subtracted the products.)

“How is multiplying decimals the same as multiplying whole numbers? How is it different?” (We use the same strategies that we used for multiplying whole numbers. We multiply different places than when we multiply whole numbers. I can use the same whole number products but then need to remember to multiply that result by 0.1 or 0.01.)

“What do you still wonder about multiplying decimals?” (Are there more strategies we can use? Does the multiplication algorithm work with decimals? Can we multiply thousandths?)

## Cool-down: Interpret Expressions (5 minutes)

### Cool-Down

For access, consult one of our IM Certified Partners.