Lesson 20

The purpose of this lesson is for students to find products of two decimal numbers where each decimal represents a number of tenths, using the place value reasoning and diagrams that were also useful for finding the product of a whole number and a decimal. Students explain why \(0.1 \times 0.1 = 0.01\) in the warm-up and this opens up one strategy for finding products like \(1.5 \times 0.7\). In particular, this can be rewritten as

\(\ (15 \times 0.1) \times (7 \times 0.1) = (15 \times 7) \times (0.1 \times 0.1)\).

So the value of \(1.5 \times 0.7\) is 105 hundredths or 1.05. A second approach uses area diagrams which students are familiar with from earlier lessons in this unit and from when they used them to find products of fractions. Decimals are an example of fractions and these diagrams are useful in this situation as well.

Activity 1: Productos de décimas