Lesson 19 Use Properties to Multiply Decimals

- Let’s interpret and evaluate multiplication expressions with decimals and whole numbers.

Warm-up Number Talk: Many Hundredths

Find the value of each expression mentally.

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

Activity 1 Card Sort: Decimal Multiplication Card Sort

Problem 1

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

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.)
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.

Problem 2

Choose one expression from each group to find the value of the expressions on cards A, B, and C.
Write at least one more expression that is equal to each of the expressions on cards A, B, and C.

Activity 2 Choose a Strategy

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

$6 \times 0.12$
$4 \times 1.4$
$5 \times 3.9$
$25 \times 0.41$

Activity 3 More Multiplication Problems

Find the value of each expression.

$35 \times 0.08$
$35 \times 0.7$
$35 \times 0.78$
$42 \times 0.66$

Practice Problem

Problem 1

Explain why each expression is equivalent to $9 \times 0.45$ .

$(9 \times 0.4) + (9 \times 0.05)$

$(9 \times 45) \div 100$

$(10 \times 0.45) - (1 \times 0.45)$
Find the value of $9 \times 0.45$ using one of the expressions or your own strategy.