Lesson 5

Choosing Strategies to Multiply

Est. Class Sessions: 2

Developing the Lesson

Part 1: Discuss Different Multiplication Strategies

Decide When to Estimate. Display the (incorrect) number sentence 21 × 39 = 8019. Tell students that one of the students in Mrs. Dewey's class used a calculator to multiply 21 × 39 and the result was 8019.

Ask:

X
  • Is this answer reasonable? (No, 8019 is almost ten times greater than the actual answer.)
  • How can you tell? (Answers may vary. Students may find the exact answer, 819, and compare it to 8019.)
  • If you did not have paper and pencil or a calculator, how would you know whether this answer was reasonable or not? (Methods may vary. Students may estimate by multiplying 20 × 40 = 800, and then compare the estimate to 8019.)

Display the When Do We Estimate? Master.

Ask:

X
  • Which situation on the display matches the example we just discussed? (Situation 1: When we want to check if an answer is reasonable.)

Draw students' attention to the other situations in the table that show when estimating makes sense. Students should be familiar with these situations and examples from previous lessons. Review the situations. Tell students that in this lesson, they will decide whether to solve problems by estimating or by finding exact answers. Being able to recognize these situations helps us to make good decisions when choosing a strategy to use.

Choose Efficient Methods. Display the problem 31 × 43. Tell students that for this problem, we want to find an exact answer. Ask students to solve it using a variety of methods from the strategies menu they created in Lesson 2 and both Multidigit Multiplication Strategies Menus from the Student Guide Reference section. For example, ask one student to use the compact method, another to use a mental-math strategy, and another to use the all-partials method.

Ask:

X
  • Which method do you use so that you know you will be accurate, that is, so you make fewer mistakes?
  • Which method is most efficient for you; that is, which method helps you get correct answers quickly?
  • Do you always use the same method, or do you use different methods for different problems?
  • How do you choose which method to use?

Students will be most efficient if they can flexibly choose methods that fit particular problems and use methods that they understand. Students may respond:

  • I can use the compact method for smaller numbers, but I like to use the all-partials method for big numbers because I am pretty sure I will get the right answer. I can keep track of the partial products more easily.
  • I like to look at a problem and see if it has numbers that make it easy to use mental math or write down just a few numbers. Then it is quicker than using all paper and pencil.
Student Guide:
Student Activity Book:
Master: