standard_description: Understand division as an unknown-factor problem. For example, find 32 ÷ 8 by finding the number that makes 32 when multiplied by 8.
problem_activity: Activity
After making an estimate of the number of sticky notes on a cabinet in the warm-up, students now brainstorm ways to find that number more accurately and then go about calculating an answer. The activity prompts students to transfer their understandings of the area of polygons to find the
surface area
of a three-dimensional object.
Students learn that the surface area of a three-dimensional figure is the total area of all its faces. Since the area of a region is the number of square units it takes to cover the region without gaps and overlaps, surface area can be thought of as the number of square units that needed to cover all sides of an object without gaps and overlaps. The square sticky notes illustrate this idea in a concrete way.
As students work, notice the varying approaches taken to determine the number of sticky notes needed to tile the faces of the cabinet (excluding the bottom). Identify students with different strategies to share later.
Launch
Arrange students in groups of 2–4. Give students 1 minute of quiet time to think about the first question and another minute to share their responses with their group. Ask students to pause afterwards.
Select some students to share how they might go about finding out the number of sticky notes and what information they would need. Students may ask for some measurements:
The measurements of the cabinet in terms of sticky notes: Tell students that the cabinet is 24 by 12 by 6.
The measurements of the cabinet in inches or centimeters: Tell students that you don’t have that information and prompt them to think of another piece of information they could use.
The measurements of each sticky note: Share that it is 3 inches by 3 inches.
If no students mention needing the edge measurements of the cabinet in terms of sticky notes, let them begin working on the second question and provide the information when they realize that it is needed. Give students 8–10 minutes for the second question.
Student Facing
Earlier, you learned about a cabinet being covered with sticky notes.
How could you find the actual number of sticky notes it will take to cover the cabinet, excluding the bottom? What information would you need to know?
Use the information you have to find the number of sticky notes to cover the cabinet. Show your reasoning.
Student Response
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Student Facing
Are you ready for more?
How many sticky notes are needed to cover the outside of 2 cabinets pushed together (including the bottom)? What about 3 cabinets? 20 cabinets?
Student Response
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Anticipated Misconceptions
Students may treat all sides as if they were congruent rectangles. That is, they find the area of the front of the cabinet and then just multiply by 5, or act as if the top is the only side that is not congruent to the others. If there is a real cabinet (or any other large object in the shape of a rectangular prism) in the classroom, consider showing students that only the sides opposite each other can be presumed to be identical.
Students may neglect the fact that the bottom of the cabinet will not be covered. Point out that the bottom is inaccessible because of the floor.
Activity Synthesis
Invite previously identified students or groups to share their answer and strategy. On a visual display, record each answer and each distinct process for determining the surface area (i.e. multiplying the side lengths of each rectangular face and adding up the products). After each presentation, poll the class on whether others had the same answer or process.
Play the video that reveals the actual number of sticky notes needed to cover the cabinet.  If students' answers vary from that shown on the video, discuss possible reasons for the differences. (For example, students may not have accounted for the cabinet's door handles. Some may have made a calculation error.)
Tell students that the question they have been trying to answer is one about the surface area of the cabinet. Explain that the
surface area
of a three-dimensional figure is the total area of all its surfaces. We call the flat surfaces on a three-dimensional figure its
faces
.
The surface area of a rectangular prism would then be the combined area of all six of its faces. In the context of this problem, we excluded the bottom face, since it is sitting on the ground and will not be tiled with sticky notes. Discuss:
“What unit of measurement are we using to represent the surface area of the cabinet?” (Square sticky notes)
“Would the surface area change if we used larger or smaller sticky notes? How?” (Yes, if we use larger sticky notes, we would need fewer. If we use smaller ones, we would need more.)
Speaking, Listening: MLR7 Compare and Connect.
As students share their strategies for determining the number of sticky notes that cover the cabinet, ask students to make connections between the various strategies. Some students will calculate the number of sticky notes that will cover each of the five faces of the cabinet and add them together. Other students may realize that opposite faces of the cabinet are congruent so it is only necessary to calculate the area of three faces of the cabinet. Encourage students to explain why both methods result in the same answer. This will promote students’ use of mathematical language as they make sense of the various methods for finding the surface area of a rectangular prism.
Design Principles(s): Cultivate conversation; Maximize meta-awareness


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