Lesson 2

The purpose of this Number Talk is to elicit strategies and understandings students have for adding multiples of 10 to two- and three-digit numbers. This activity helps students extend their fluency with adding multiples of 10 to two-digit numbers to their work in the lesson activities with adding and subtracting multiples of 10 and 100 within 1,000. As students share their thinking, consider recording on an open number line.

• 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 on an open number line.
• Keep expressions and work displayed.
• Repeat with each expression.

• “How did you find the value of these expressions?” (We noticed we could count on by tens. We added the number of tens in each one. The number in the ones place didn’t change.)

The purpose of this activity is to add and subtract three-digit numbers and multiples of 10 or 100 using base-ten blocks and equations. The structure of the base-ten blocks encourages students to notice that hundreds are added to or subtracted from hundreds and tens are added to or subtracted from tens (MP7). To help students focus on this structure, the numbers are chosen so that the addition and subtraction of tens can take place without needing to compose or decompose a hundred. 

If you do not have enough hundred blocks for each group of 2 students, students may work in larger groups.

• Groups of 2
• Give each group base-ten blocks and a number cube.

• “Work with your partner to show each number with base-ten blocks.”
• “Take turns rolling the number cube to see how many tens or hundreds to add or subtract.”
• 10 minutes: partner work time
• Monitor for students to share their equations for the number of hundreds they subtract from 805.

• Invite previously identified students to share equations or display
  • \(805 - 400 = 405\)
• Display a base-ten diagram, or, if possible, allow students to demonstrate with base-ten blocks.
• “How does the equation connect to the diagram or blocks?” (You take away 4 hundreds from the blocks and you’re left with 4 hundreds and 5 ones.)
• “What did you notice about how the value of the blocks changed?” (There are fewer blocks. The hundreds place changed because you subtracted hundreds.)

The purpose of this activity is for students to add and subtract multiples of 10 and 100 within 1,000. Students interpret situations with base-ten blocks and base-ten diagrams and use equations to represents sums and differences. In the activity synthesis, students relate the situation, base-ten diagram, and equation and notice, through repeated reasoning, that when adding and subtracting, one can add or subtract hundreds and hundreds and tens and tens (MP7, MP8).

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

• “These students were using base-ten blocks to find the value of sums and differences.”
• “For each problem, represent the student’s value in a way that makes sense to you. You can use base-ten blocks, diagrams, or equations.”
• 10 minutes: partner work time
• For Priya and Jada’s blocks, monitor for students who:
  • write equations to show the sums and differences.
  • use base-ten diagrams to show subtraction.

If students draw diagrams that do not match the situation or write equations that are not true, consider asking:

• “Can you explain what you did?”
• “Do you notice any patterns when you add or subtract hundreds and hundreds? Tens and tens?”

• Invite a student to share their representation and equation for Jada’s blocks. If no students use a base-ten diagram, select a student to share how they use base-ten blocks, and use a base-ten diagram to record their steps.
• “How does the base-ten diagram show Jada’s blocks?” (The blocks show how many Jada started with. The lines show the 3 hundreds that were taken away.)
• Display \(482 - 300 = 182\)
• “How does the equation show Jada’s blocks?” (482 is what Jada started with and 300 is the 3 hundreds she gave away. 182 is how much she has left.)