Lesson 3

Students have previously seen tape diagrams used to represent equal-sized groups. This warm-up gives students a chance to create tape diagrams to represent division expressions in a scaffolded way. Each tape is started on a grid and pre-labeled with the known quantity. Each grid square represents 1.

Arrange students in groups of 2. Give students 2 minutes of quiet work time and another minute to share their diagrams with their partner.

Select a few students to share their diagrams and answers. After each person shares, poll the class to see if others agree or disagree. 

Discuss questions such as:

• “How did you know how large each part of the diagram should be?”
  (In the first question, the length of the tape represents 10, and there are 10 grid squares, so each grid represents 1. Since the size of each group is \(2\frac12\), each part needs to have \(2\frac12\) squares.)
• “In the second question, we see three groups of 2 and an extra square of 1. How did you know that the 1 is \(\frac12\) of a group and not \(\frac17\) of a group?” (The question asks “how many groups of 2 . . .,” so the size of each group is 2, not 7.)

Tell students they will use tape diagrams to help solve other division problems.

In this lesson, students learn to represent “how many groups?” or “how many of this in that?” questions by drawing tape diagrams on a grid.

Students continue to write multiplication and division equations to make sense of given situations, but here, they also think about the reasonableness of their answers. They see that the solution to a division problem can be checked using the corresponding multiplication equation.

Some students may write answers as fractions and others as mixed numbers. Both are acceptable. Depending on the situation, one may be more useful than the other. For example, in a measurement situation, a mixed number is easier to interpret, but if we need to perform further calculations with an answer, a fraction is easier to work with. In the course of the unit, students should become comfortable with both forms and be flexible in using them.

Arrange students in groups of 3–4. Tell students that one way to solve “how many groups?” and “how many of this in that?” problems is by drawing tape diagrams.

Give students 2–3 minutes to discuss the first question and write their response. Follow with a brief whole-class discussion. Select 1–2 students to explain how a tape diagram shows us how many \(\frac 23\)s are in 1.

Then, give students 8–10 minutes of quiet work time to complete the rest of the task. Ask them to discuss their diagrams only after attempting at least 2 of the 3 remaining questions. Provide access to colored pencils. Some students may find it helpful to identify whole groups and partial groups on a tape diagram by coloring. 

Focus the whole-class discussion on two goals: reflecting on the process of creating and using the diagrams, and discussing how we can check our solutions.

For each problem in the second question, select a student to share their response and ask how many others had the same diagram.
If students’ diagrams cannot be easily displayed for all to see, consider showing the ones in the Possible Responses. To help students reflect on their process, discuss:

• “How did you begin the diagram? How did you know how to partition the pieces in the diagram?”
• “Which of the two equations—multiplication or division—was helpful in setting up the diagram? How so?”
• “How did you determine how many groups there are?”
• Reinforce the idea that the size of 1 group is what we use as the unit for counting and to find out how many groups there are.

To prompt students to think about the reasonableness of their answers, ask: “How would you know if your answer is correct?” If not mentioned by students, point out how to use multiplication to check their solution to the division problem. For example:

• We wrote \({?} \boldcdot \frac 23 = 1\) to represent “how many \(\frac 23\) are in 1?”.
• We found the answer to be \(1\frac12\), so we can substitute \(1\frac12\) for the “?” and see if \(1\frac12 \boldcdot \frac23\) is indeed 1.
• \(1\frac12 = \frac 32\), so we can rewrite that expression as \(\frac 32 \boldcdot \frac 23\).
• \(\frac 32 \boldcdot \frac 23 = 1\), so the answer is correct.

For classes using the digital materials, consider demonstrating an applet to represent a division problem https://ggbm.at/atUteypU. This video shows how to use the tool to answer the question “How many \(\frac12\) are in 3?” https://vimeo.com/184879045.

In this activity, students apply the reasoning and strategies from the previous activity to solve division problems in context. Though the instructions do not prompt students to draw tape diagrams, students may find them to be a handy option.

Students may also choose to express each whole-number or mixed-number dividend as a fraction. For example, they may express the 6 inches as \(\frac{48}{8}\) inches and then see how many \(\frac38\)s are in \(\frac{48}{8}\). As students work, notice the different strategies they use.

Keep students in the same groups. Give students 8–10 minutes of quiet work time followed by a few minutes to discuss their work with their group.

Some students may wish to use graph paper to draw tape diagrams and colored pencils to mark up parts of the diagrams. Provide access to geometry toolkits. Encourage students to think about the tools and strategies at their disposal and to check their solutions using multiplication.

Because the activity is intended to help students reason about division problems quantitatively and abstractly, consider wrapping it up by asking students to write a brief reflection about their reasoning. Display the following prompts for all to see and ask students to write their response on an index card or a sheet of paper so it can be collected.

“A friend is unsure what \(2 \div \frac45\) means and isn't sure how to find its value. How would you help your friend make sense of the expression? How do you think about it? Share two ways that you find helpful for reasoning about an expression like this.”