Lesson 9

The purpose of this Number Talk is to elicit strategies and understandings students have for making a ten when adding a one-digit number to three-digit numbers. In the synthesis, the focus is on how students look for ways to make ten. These understandings help students develop fluency and will be helpful in the lesson as students add 2 three-digit numbers and consider composing units.

• Display one expression.
• “Give me a signal when you have an answer and can explain how you got it.”
• 1 minute: quiet think time

• Record answers and strategies.
• Keep expressions and work displayed.
• Repeat with each expression.

• “What do you notice about the sums?” (I made a ten to find the sum. They all show adding some ones to three-digit numbers. There’s a pattern in the ones—0 ones, 5 ones, 0 ones, 5 ones.)
• “Why are \(528 + 2\) and \(487 + 3\) helpful in finding the value of the other expressions?” (We know they make a new ten, so it helped me think about how I could decompose a number to help make a ten.)

The purpose of this activity is for students to practice adding within 1,000 with an emphasis on adding by place. Throughout the activity, students have opportunities to explain their methods and representations to peers. In the synthesis, students have opportunities to question and critique the work of their peers as they compare different methods for adding three-digit numbers and different representations that show composing units when adding by place (MP3).

This activity uses MLR8 Discussion Supports. Advances: conversing

• Groups of 2
• Give students access to base-ten blocks.

• “We are going to practice adding 2 three-digit numbers. You can find the value of each sum in any way that makes sense to you. After some time working independently, you can continue working with a partner.”

MLR8 Discussion Supports

• Display sentence frames to support students when they explain their strategy:
  • “First, I _____ because . . .”
  • “I noticed _____ so I . . .”
• 5 minutes: independent work time
• 4 minutes: partner work time
• “Choose one expression and share how you found the value of the sum with a new partner.”
• 3 minutes: partner discussion
• Monitor for students who use base-ten blocks, base-ten diagrams, or other representations that clearly show adding by place.

If students lose track of the tens or hundreds they compose (for example, \(384 + 409 = 783\)) or do not compose a ten or hundred, prompt students to explain their thinking and any representations they use. Consider asking:

• “How does your representation show that you composed a new unit?”
• “What could you add to your representation to help you keep track of any tens or hundreds you compose?”
• “How could you use base-ten blocks to show your thinking?”

• Invite 2–3 previously identified students to share their representations and how they found the value of the sums.
• “How are these methods the same? How are they different?”

The purpose of this activity is for students to analyze two methods for finding a sum within 1,000. Students use what they know about adding by place and composing units to determine which method shows the correct value of the sum. Students discuss why one method did not work and ways the method could be adjusted to find the correct sum (MP3).

• Groups of 2
• Give students access to base-ten blocks.

• “Noah and Kiran were asked to find the value of \(267 + 338\). Take a minute to look at each student’s work.”
• 1–2 minutes: quiet think time
• “Discuss how Noah and Kiran’s work is the same and how it is different. Then explain which student found the correct value of the sum. Show how you know.”
• 10 minutes: partner work time
• Monitor for groups that match Kiran’s equations to Noah’s diagram to explain where Noah could improve his work.

• “How is Noah and Kiran’s work the same? How is it different?” (They both show adding by place. Noah used a base-ten diagram and Kiran used equations. They found different values. Noah forgot to add the ten he made from the ones. He had enough tens to make a hundred.)
• “Which student found the correct value for \(267 + 338\)? How do you know?”
• Invite previously identified students to share.
• Consider asking:
  • “Where do you see the step Noah missed in Kiran’s work?”