Lesson 7

Multiplying More Than Two Factors

Est. Class Sessions: 1

Developing the Lesson

Part 2: Multiplying by Multiples of 10

Use Skip Counting, Money, and Grouping. The Multiplying by Multiples of 10 pages in the Student Activity Book use the skills practiced in Part 1 to develop strategies for multiplying numbers by multiples of 10 in preparation for multiplication of two-digit numbers.

Review the definition of multiples of 10 that is given at the top of the page so that students understand the goal of the pages. Then, encourage students to work with a partner to solve the problems in Questions 1–12. These questions review three multiplication strategies: skip counting, money, and grouping.

Then ask:

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  • Which strategy makes the most sense to you? Which do you understand best? Why?
  • Explain how to solve 9 × 20 (Question 7B) using skip counting. Did you get the same answer as you did using money? (Skip count by 20 nine times to get 180: 20, 40, 60, 80, 100, 120, 140, 160, 180.)
  • Explain a way to solve 50 × 7 (Question 5) using money. Did you get the same answer as you did skip counting? (Possible response: Two 50¢ pieces are $1. Six 50¢ pieces are $3. One more 50¢ is $3.50, 50 × 7 = 350.)
  • Talk with another pair of students. Compare how you used grouping to solve 30 × 4 (Question 10A). What three factors did you use? How did you group them? (Possible response: (3 × 10) × 4, 3 × (10 × 4))

Then discuss the patterns students see in their answers when they multiply by multiples of 10 (Question 12). Students will likely express the patterns by giving examples such as:

“For 90 × 2, just multiply the 9 and 2 to get 18 and then just add a 0.”

“Multiply the first digit in the multiple of 10 by the other number, then add a 0.”

Ask students to think for a minute about why they can add a zero:

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  • When you add a zero, what does that tell you about a number? (Possible response: That number is a multiple of 10. So, when you multiply by tens, you get a multiple of 10.)
  • How many zeros are at the end of the answer to 50 × 6? Why? (2 zeros. 50 × 6 = 300. 5 × 6 = 30, then when you multiply that by 10 you get 300.)
Student Guide:
Student Activity Book: 79 80
Answers
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Master: