Lesson 12

The purpose of this warm-up is to elicit the idea that addition and subtraction are related operations, which will be useful when students solve subtraction story problems with unknown addends. 

• Groups of 2
• Display the image.
• “What do you notice? What do you wonder?”
• 1 minute: quiet think time

• “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Share and record responses.

• “Use these equations to explain how addition can help with subtraction.” (If you know an addition fact like \(6 + 8 = 14\), then you also know two subtraction facts, \(14 - 8 = 6\) and \(14 - 6 = 8\).)

The purpose of this activity is for students to make sense of and solve a new type of story problem, Compare, Smaller Unknown, using the Three Reads instructional routine. The routine helps students make sense of the quantities in the problem and their relationship before they try to represent and solve the problem (MP2).

In the activity synthesis, students make sense of different representations of the problem. They may use connecting cubes or drawings to represent their thinking as they solve the problem. Since this is a Compare problem, monitor for representations in which each bracelet length is represented, similar to what students will see in the lesson synthesis.

This activity uses MLR6 Three Reads. Advances: reading, listening, representing

• Groups of 2
• Give students access to connecting cubes in towers of 10 and singles.

MLR6 Three Reads

• Display only the problem stem, without revealing the question.
• “We are going to read this problem three times.”
• 1st Read: “Priya and Han are comparing the lengths of their friendship bracelets. Han’s bracelet is 14 cubes long. The length of Priya’s bracelet is 4 cubes fewer than Han’s bracelet.”
• “What is this story about?”
• 1 minute: partner discussion
• Listen for and clarify any questions about the context.
• 2nd Read: “Priya and Han are comparing the lengths of their friendship bracelets. Han’s bracelet is 14 cubes long. The length of Priya’s bracelet is 4 cubes fewer than Han’s bracelet.”
• “What can be counted or measured?" (The length of Priya's bracelet. The length of Han's bracelet. The difference in length between the two bracelets.)
• 30 seconds: quiet think time
• 1 minute: partner discussion
• Share and record all quantities.
• 3rd Read: Read the entire problem, including the question, aloud.
• “What are different ways we can solve this problem?” (use connecting cubes to represent the bracelets, draw a picture, think about the numbers)
• 30 seconds: quiet think time
• 1–2 minutes: partner discussion

• “Work with your partner to represent and solve the problem. Show your thinking using drawings, numbers, or words.”
• 3–5 minutes: partner work time
• Monitor for students who use different representations, including a drawing that shows both quantities.

• Invite previously identified students to share their representations.
• “How did _____ represent the problem? Where do you see the answer to the question in their work?”

The purpose of this activity is for students to make sense of and solve a Compare, Bigger Unknown story problem. The numbers are intentionally the same as the problem in the previous activity. During the lesson synthesis, students analyze a representation and describe how it can represent both story problems (MP8).

• Groups of 2
• Give students access to connecting cubes in towers of 10 and singles.
• “Now you are going to solve another problem about Han’s and Priya’s bracelets.”

• Read the task statement.
• 5 minutes: partner work time
• Monitor for students who notice how this problem is related to the problem in the previous activity.

• Invite previously identified students to share what they noticed.
• Invite students to share how they solved the problem.

The purpose of this activity is for students to learn stage 3 of the Write Numbers center. Students count by 1 and choose whether to count forward or backward. They take turns writing the next 1, 2, or 3 numbers in the sequence. The player who writes the last number on the number path wins. Gameboards go from 89–110, 95–116, and 99–120.

Put each gameboard in a sheet protector.

• Groups of 2
• Give each group the gameboards and a dry erase marker.
• “We are going to learn a new way to play Write Numbers.”
• “This time, the gameboards have larger numbers so you can practice writing numbers above 100. Decide which gameboard to begin with and decide whether to start with the smaller number and count forward, or start with the larger number and count backward. Take turns writing the next one, two, or three numbers on the path. Remember, the person who writes the last number wins.”

• 10 minutes: partner work time

• Play one round with the class. Discuss how they decide whether to write one, two, or three numbers.