Lesson 2

The purpose of this number talk is to elicit strategies and understandings students have for dividing a fraction by a fraction. Later in this lesson, students will need to be able to divide a fraction by a fraction to solve problems in contexts.

Four problems are given. It may not be possible to share every strategy. Consider gathering only two or three different strategies per problem, saving most of the time for the final question.

Reveal one problem at a time. Give students 30 seconds of quiet think time for each problem and ask them to give a signal when they have an answer and a strategy. Keep all previous problems displayed throughout the talk. Follow with a whole-class discussion.

Students may get stuck trying to remember a procedure to divide fractions or may also think the answer to each problem is just the whole number divided by 3. Help students reason about dividing a whole number by a unit fraction by asking what number if divided into thirds gives us an answer of ___ (5, 2, \(\frac12\), \(2\frac12\))?

If students get stuck on the last problem, help them see that previous problems can be used to figure out an answer to this last one. Since \(2\frac12\) is half of 5 the answer is going to be half of the answer to \(5\div\frac13\). They may also apply the distributive property to use the answer to \(2\div\frac13\) and \(\frac12\div\frac13\) to figure out the answer.

Ask students to share their strategies for each problem. Record and display their explanations for all to see. To involve more students in the conversation, consider asking:

• “Who can restate ___’s reasoning in a different way?”
• “Did anyone have the same strategy but would explain it differently?”
• “Did anyone solve the problem in a different way?”
• “Does anyone want to add on to _____’s strategy?”
• “Do you agree or disagree? Why?”

The purpose of this activity is to review different strategies for working with ratios and to prepare students to use these strategies with ratios involving fractions. The activity also foreshadows percentages by asking about the distance traveled in 100 minutes.

Monitor for different strategies like these:

• divide \(\frac{15}{2}\div 6\) to find the distance traveled in 1 minute, and then multiply it by 100.
• draw a double number line.
• create a table of equivalent ratios.

Depending on their prior learning, students might lean towards the first strategy.

Give students 3 minutes of quiet work time. Encourage them to find more than one strategy if they have time. Follow with whole-class discussion around the various strategies they used.

Students might calculate the unit rate as \(6\div \frac{15}{2}\). Ask students what this number would mean in this problem? (This number means that it takes \(\frac45\) of a minute to travel 1 kilometer.) In this case, students should be encouraged to create a table or a double number line, since it will help them make sense of the meaning of the numbers.

Select students to share the strategies they used. To the extent possible, there should be one student per strategy listed. If no students come up with one or more representations, create them so that students can compare and contrast.

• Divide (\(\frac{15}{2}\div 6\)) to find the number of kilometers traveled in 1 minute, and multiply by 100
• Double Number Line
• Table

Display strategies for all to see throughout the discussion.

Help students connect the strategies by asking:

• Was there a place in your solution where you calculated \(\frac{15}{2}\div 6\)?
• How can we see this value being used in the double number line? Table?

The purpose of this activity is to provide another context that leads students to calculate a unit rate from a ratio of fractions. This work is based on students’ work in grade 6 on dividing fractions.

Students notice and wonder about two statements and use what they wonder to create questions that are collected for all to see. Each student picks a question secretly and calculates the answer, then shares the answer with their partner. The partner tries to guess the question. Most of the time in this activity should be spent on students engaging in partner discussion.

Arrange students into groups of 2. Display the two statements for all to see. Ask students to write down what they notice and wonder, and then use what they wonder to come up with questions that can be answered using the given information. Create a list of questions and display for all to see. Here are suggested questions to listen for:

• Who ran faster, Noah or Lin?
• How far would Lin run in 1 hour?
• How far did Noah run in 1 hour?
• How long would it take Lin to run 1 mile at that rate?
• How long would it take Noah to run 1 mile at that rate?

The warm-up was intended to remind students of some strategies for dividing fractions by fractions, but students may need additional support working with the numbers in this task.

Students might have a hard time guessing their partner's question given only the answer.  Ask their partners to share the process they used to calculate the solution, they might leave out numbers and describe in general the steps they took to find the answer first. If their partner is still unable to guess the question, have them share the specific number they used. If they need additional support to guess the question, have their partner show them their work on paper (without sharing the question they answered) and see if this helps them figure out the question.

After both partners have a chance to guess each other’s question, ask a few different students to share their strategies for guessing which question their partner answered.

The purpose of this activity is to provide a context where a ratio of fractions arises naturally, and students need to find an equivalent ratio to solve the problem. The ratio \(2 \frac12: 1 \frac34\) is equivalent to \(10:7\), so a scaled copy of the Mona Lisa that is 10 inches by 7 inches would fit on the cover of the notebook. Other answers are possible. Some students might try to find the biggest possible copy that will fit on the cover, which would result in a different scale factor.

The digital version of the student materials includes an applet so that students can experiment with the context, because there are many related measurements within the context that can be hard to visualize. For example, the applet makes it clear that you can't simply scale down the Mona Lisa and make it perfectly fit on the notebook, since the notebook and the Mona Lisa are not scaled copies of each other. It also serves to remind students that the length and width of the Mona Lisa have to be scaled by the same factor, or the image becomes distorted. 

Students discuss how they found their solution with a partner and determine if the scale factors they came up with are reasonable. Students must think about whether it makes sense to try to scale the picture so that it fills as much of the page as possible, or whether it makes more sense to leave room for a title. As students discuss with their partner, identify pairs of students who have a good argument that a certain scale factor makes more sense to use than another. Ask these students to share during the whole class discussion.

Students might get stuck thinking the scaled copy needs to measure 11 inches by 9 inches.  Ask students: 

• Does the copy of the painting have to cover the entire notebook?
• What are some other options if the image doesn't cover the entire notebook?
• What if the image is bigger than the notebook cover? What if it is smaller?

Select previously identified students to share the arguments they had with their partners.  Some guiding questions:

• What scale factor did you and your partner agree upon? How did you both agree upon this?
• Were there any similarities between the methods you and your partner used?  Were there any differences?