Lesson 6
Methods for Multiplying Decimals
Lesson Narrative
In this lesson, students continue to develop methods for computing products of decimals, including using area diagrams. They multiply decimals by expressing them as fractions, or by interpreting each decimal as a product of a whole number and a power of 10 and \(\frac{1}{10}\). To multiply \((0.25) \boldcdot (1.6)\), for example, students may first multiply 0.25 by 100 and 1.6 by 10 to have whole numbers 25 and 16, multiply the whole numbers to get 400, and then multiply 400 by \(\frac {1}{1,000}\) to invert the initial multiplication by 1,000. They may also think of 0.25 and 1.6 as \(\frac {25}{100}\) and \(\frac {16}{10}\), multiply the fractions, and then express the fractional product as a decimal.
In earlier grades, students used the area of rectangles to represent and find products of whole numbers and fractions. Here they do the same to represent and find products of decimals. They see that a rectangle that represents \(4 \boldcdot 2\), for instance, can also be used to reason about \((0.4) \boldcdot (0.2)\), \((0.004) \boldcdot (0.002)\), or \(40 \boldcdot 20\) because they all share a common structure. In this lesson, students extend their understanding of multiplication of fractions and multiplication using area diagrams by using previous methods to multiply any pair of decimals.
Learning Goals
Teacher Facing
 Interpret different methods for computing the product of decimals, and evaluate (orally) their usefulness.
 Justify (orally, in writing, and through other representations) where to place the decimal point in the product of two decimals with multiple nonzero digits.
Student Facing
Let’s look at some ways we can represent multiplication of decimals.
Learning Targets
Student Facing
 I can use area diagrams to represent and reason about multiplication of decimals.
 I know and can explain more than one way to multiply decimals using fractions and place value.
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