Lesson 3

The purpose of this Number Talk is to elicit strategies and understandings students have for adding three-digit numbers. These understandings help students develop fluency and will be helpful later in this lesson when students are to use strategies based on place value and properties of operations to add within 1,000.

• 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 strategy.
• Keep expressions and work displayed.
• Repeat with each expression.

• “How did you use place value to find the value of each sum?” (I added hundreds with hundreds and tens with tens.)
• Consider asking:
  • “Who can restate _______ 's reasoning in a different way?”
  • “Did anyone have the same strategy but would explain it differently?”
  • “Did anyone approach the problem in a different way?”
  • “Does anyone want to add on to____’s strategy?”

The purpose of this activity is for students to add within 1,000 using any strategy that makes sense to them. The expressions in this activity give students a chance to use different strategies, such as adding hundreds to hundreds, tens to tens, and ones to ones, reasoning with numbers close to a hundred, or using a variety of representations. Students who use base-ten blocks or draw number line diagrams choose appropriate tools strategically (MP5).

• Groups of 2
• Give students access to base-ten blocks.
• “Take a minute to think about how you could find the value of each sum.”
• 1 minute: quiet think time
• Share responses.

• “Work with your partner to add these numbers in any way that makes sense to you. Explain or show your reasoning.”
• 5-7 minutes: partner work time
• Monitor for an expression for which students use a variety of representations, such as:
  • Using base-ten blocks
  • Drawing a number line
  • Writing their reasoning in words
  • Writing equations
• Identify students using different representations to share during synthesis.

• Select previously identified students to display their work side-by-side for all to see.
• “Which representations show the same idea or help us find the sum the same way?” (The base-ten blocks and equations show adding hundreds to hundreds, tens to tens, and ones to ones. The number line and the words both added on the second number to the first number in parts.)

The purpose of this activity is for students to see that they can start adding from the largest place-value unit or from the smallest and still get the same sum. This understanding prepares students to use the standard algorithm for addition, which calls for starting with the ones.

• Groups of 2
• “Take a minute to look at Clare and Andre’s work. Think about how their work is alike and how it's different.”
• 1 minute: quiet think time

• “Talk with your partner about what’s different about Clare and Andre’s work and what’s the same.”
• 3-5 minutes: partner discussion

• Invite students to share their responses.
• Consider asking:
  • “If you were to describe the steps that Andre took to add and the steps that Clare took to add, what would they be?” (Andre added the hundreds, added the tens, added the ones, then added up all the parts to find the sum. Clare added the ones, added the tens, added the hundreds, then added up the parts to find the sum.)
  • “How is it that Andre started with the hundreds and Clare started with the ones, but they both found the same sum?” (It doesn’t matter the order that we add the numbers. If they’re the same numbers we’ll get the same sum.)