standard_description: Interpret multiplication as scaling (resizing), by:
- Explaining why multiplying a given number by a fraction greater than 1 results in a product greater than the given number (recognizing multiplication by whole numbers greater than 1 as a familiar case); explaining why multiplying a given number by a fraction less than 1 results in a product smaller than the given number; and relating the principle of fraction equivalence a/b = (n×a)/(n×b) to the effect of multiplying a/b by 1.
- Comparing the size of a product to the size of one factor on the basis of the size of the other factor, without performing the indicated multiplication.
problem_activity: Narrative
The purpose of this activity is for students to write equivalent expressions in order to find the volume of a figure composed of two right rectangular prisms. Students decompose the figure in two different ways, and write matching expressions to find the volume. For extra support, provide students with colored pencils to shade the two parts of the prism before finding the side lengths they need to calculate the volume.
Monitor and select a student with each of the following strategies to share in the synthesis:
decomposed the figure into a prism with the side lengths 4 ft by 4 ft by 3 ft and a prism with the side lengths 10 ft by 4 ft by 3 ft and wrote this expression (or one written in a different order) to represent the volume:
\((4 \times 4 \times 3) + (10 \times 4 \times 3)\)
decomposed the figure into a prism with the side lengths 4 ft by 8 ft by 3 ft and a prism with the side lengths 6 ft by 4 ft by 3 ft and wrote this expression (or one written in a different order) to represent the volume:
\((4 \times 8 \times 3) + (6 \times 4 \times 3)\)
Action and Expression: Internalize Executive Functions.
Invite students to plan a strategy, including the tools they will use, for finding the volume of the figures. If time allows, invite students to share their plan with a partner before they begin.
Supports accessibility for: Organization, Memory, Attention
Launch
Groups of 2
Activity
10 minutes: individual work time
5 minutes: partner discussion
As students work, consider asking, “Why did you choose to decompose the prism that way?”
Student Facing
Find the volume of the figure by decomposing the figure 2 different ways. Show your thinking. Organize it so it can be followed by others.
[IMG]
[IMG]
For each way you decomposed the figure, write an expression that represents the volume.
Mai used this expression to find the volume of the figure:
\((10 \times 8 \times 3) - (6 \times 4 \times 3)\)
.
Use the diagram to interpret Mai's expression. Show your thinking. Organize it so it can be followed by others.
[IMG]
Student Response
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Activity Synthesis
Ask the two selected students to display their work side by side for all to see.
“How are the diagrams the same? How are they different?”
“How do the expressions relate to the diagrams?”
Display:
\((10 \times 8 \times 3) - (6 \times 4 \times 3)\)
“How does this expression represent the volume of the prism?” (The larger rectangular prism has the side lengths
\(10 \times 8 \times 3\)
cubic feet. We can subtract a rectangular prism with the side lengths
\(6 \times 4 \times 3\)
cubic feet.)
“What is the value of
\((10 \times 8 \times 3) - (6 \times 4 \times 3)\)
?” (168)


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