Lesson 12

This number talk encourages students to think carefully about the numbers in a computation problem and rely on what they know about structure, patterns, and properties of operations to mentally solve it. The reasoning helps students develop fluency and will support students in calculating products and quotients in upcoming work.

Give students 2 minutes of quiet think time and ask them to give a signal when they have an answer and a strategy. Follow with a whole-class discussion.

When multiplying \(19\boldcdot 14\), students may only multiply the tens digits and multiply the ones digits and add them to get 136. Ask these students to estimate an answer for the problem and consider whether their answer makes sense.

Ask students to share their strategies for each problem. Record and display their explanations for all to see. Ask students if or how the factors in the problem impacted their strategy choice. To involve more students in the conversation, consider asking:

• “Who can restate ___’s reasoning in a different way?”
• “Did anyone solve the problem the same way 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?”

In this activity, students use division to solve problems involving lengths. No methods are specified for any of the questions, so students need to choose an appropriate strategy. 

The info gap structure requires students to make sense of problems by determining what information is necessary, and then to ask for information they need to solve it. This may take several rounds of discussion if their first requests do not yield the information they need (MP1). It also allows them to refine the language they use and ask increasingly more precise questions until they get the information they need (MP6).

Here is the text of the cards for reference and planning:

Arrange students in groups of 2. In each group, distribute the first problem card to one student and a data card to the other student. After debriefing on the first problem, distribute the cards for the second problem, in which students switch roles.

Some students may not know what is meant by the “spine” of a book. Consider holding up a book and pointing out where its spine is.

If students struggle to represent the situations mathematically, suggest that they draw diagrams to represent the situations. They could start with sketches of the objects and then move toward other simpler or more abstract representations as they make better sense of the problems.

Select previously identified students to share their solutions and reasoning for each question. Start with students who used the more involved methods and move toward the more efficient ones. Record the approaches for all to see.  

Highlight the connections between the different methods (e.g., between diagrams and equations, between a multiplication equation one student wrote and a division equation another person wrote for the same situation, etc.).

In this activity, students practice performing division of fractions and using it to solve multiplicative comparison problems. The activity extends the work students have done earlier in the unit. In the Fractions of Ropes activity (in Lesson 7), students use diagrams to reason about how many times as long one rope is compared to another. Having had more experience in interpreting division situations and having learned a division algorithm, students can solve a wider range of problems that involve a greater variety of fractions. Minimal scaffolding is given here, so students need to decide what representations or strategies would be fruitful.

Arrange students in groups of 4. Give students 1–2 minutes of quiet time to think about and draw a diagram for each question in the first problem. Ask them also to think about and write two equations that can represent the two questions. Afterward, give each group 2 minutes to compare their diagrams and equations.

Invite a couple of students to share their diagrams and equations. Ask the class whether they agree that the diagrams and equations represent the questions. Once students agree that the representations are appropriate, give them 8–10 minutes to complete the activity, either independently or collaboratively with their group. Encourage students to estimate the answer before calculating and to check their quotients using multiplication.

If students have trouble drawing and using a diagram to compare lengths, ask them to revisit the Fractions of Ropes activity (in Lesson 7) and use the diagrams there as examples. Suggest that they try drawing a diagram on graph paper, as the grid could support them in drawing and making sense of the fractional lengths.

Consider giving students access to the answers so they can check their work. Much of the discussion will have happened in small groups. If time permits, reconvene as a class to discuss the last set of questions and the different ways they were represented and solved. 

This optional activity gives students another opportunity to solve a contextual problem using what they know about fractions, relationships between multiplication and division, and diagrams. Students observe a photograph of two paper rolls of differing lengths and estimate the relationship between the lengths. The photograph shows that the longer roll is about \(2\frac12\) or \(\frac52\) times as long as the shorter roll. Students use this observation to find out the length of the shorter roll.

The two paper rolls are from paper towels and toilet paper. If possible, consider providing one of each roll to each group of students so they can physically compare their lengths in addition to observing the picture.

As students work, notice the different starting equations or diagrams they use to begin solving the problems. Ask students using different entry points to share later.

Keep students in groups of 4. Ask students to keep their materials closed. Display the image of the paper rolls for all to see. Give students 1–2 minutes to observe the picture. Ask them to be prepared to share at least one thing they notice and one thing they wonder. Then, invite students to share their observations and questions. If no students mention the relationship between the lengths of the rolls, ask them questions such as:

• “What do you notice about the lengths of the paper rolls or the relationships between those lengths?”
• “What questions can you ask about the lengths of the rolls?”
• “What information would you need to answer these questions?”

Then, give students 7–8 minutes of quiet time to complete the questions.

Students might estimate the relationships between the lengths of rolls by rounding too much. For example, they might say that the length of the shorter roll is \(\frac{1}{3}\) the length of the longer roll, or that the longer roll is twice as long as the shorter roll. If this happens, ask students to take a closer look and make a more precise estimate. Suggest that they divide the larger roll into smaller segments, each of which matches the length of the shorter rolls.

Invite previously identified students to share their strategies for finding the length of the short roll. Display their diagram and record their reasoning for all to see.

To find the length of the short roll, some students may use \(11 \frac 14 \div \frac 52 = ?\) and others \(\frac 25 \boldcdot \frac{45}{4} = ?\), depending on how they view the relationship between the rolls. Highlight the idea that to find the length of the short roll, one way is to partition the length of the large roll into 5 equal pieces and find that length and multiply it by 2, because the length of the shorter roll is about \(\frac25\) of that of the longer roll. This is an opportunity to reinforce the structure behind the division algorithm.

If appropriate, discuss the merits of writing the numbers as fractions versus as mixed numbers. Mixed numbers such as \(2\frac12\) and \(11\frac14\) are easier to visualize but \(\frac52\) and \(\frac{45}{4}\) are easier to work with. In fact, we have to use the fractions (rather than mixed numbers) to easily multiply. Explain that two forms serve different purposes and that it is helpful to be able to change from one to the other depending on what we aim to do. When writing them as solutions to problems, both forms are mathematically correct.