Lesson 15

The purpose of this Choral Count is for students to practice counting back by 10 and notice patterns in the count. These understandings help students develop fluency and will be helpful later in this lesson when students will need to be able to decompose hundreds. 

• “Count back by 10, starting at 590.”
• Record as students count.
• Stop counting and recording at 390.

• “What patterns do you see?”
• 1–2 minutes: quiet think time
• Record responses.

• “What comes after 390 if we keep counting back by 10? What comes after 300?”

The purpose of this activity is for students to use their knowledge of base-ten diagrams and place value to make sense of a written method. They describe how numbers, words, and equations can be used to represent the steps they have used with other representations (MP2). Students will explore other ways to use equations, including the standard algorithm, in future grades. Although some grade 2 students may begin to use recording methods like the standard algorithm, it is important to encourage students use the methods that make the most sense to them.

• Groups of 2
• Give students access to base-ten blocks.
• Display Elena’s work.
• “Take a minute to make sense of Elena’s subtraction.”
• 1–2 minutes: quiet think time
• “Discuss Elena’s work with your partner.”
• 1–2 minutes: partner discussion
• Share and record responses. Highlight responses that describe the numbers Elena is subtracting and how Elena shows decomposing units.

• “Elena is finding the value of \(726-558\). Use base-ten blocks or a base-ten diagram to show Elena’s steps. Then finish Elena’s work. If you have time, work together to show a different way Elena could use numbers or equations to show her steps.”
• 7 minutes: partner work time
• Monitor for student examples that record in a way similar to Elena, but use numbers to show the value of each place.

If students rely on one way to show subtracting by place with numbers or equations, consider asking:

• “How did Elena represent the value of each digit in 726 and 558?”
• “What is another way you could represent the value of the digits and show subtracting by place?”

• Display a student representation of Elena’s steps with a base-ten diagram and 1–2 other representations that use numbers and equations.
• “How are these representations the same? How are they different?” (They all show decomposing a hundred and a ten before you subtract. They all show subtracting hundreds from hundreds, tens from tens, and ones from ones. They use different ways to represent each unit. One uses base-ten diagrams, one uses numbers and words, and one just represents the value of each place.)

The purpose of this activity is for students to practice subtracting within 1,000. Each student starts with a card with a three-digit number on it. Students walk around the room and find a student with a different number. They find the difference between their numbers and write an equation to represent the difference. As time allows, students find a new partners. The numbers on the cards were chosen so any pair of numbers would require decomposing a hundred and a ten when subtracting by place. However, in addition to monitoring for ways students try written methods for subtracting by place, monitor for the ways students use what they know about the relationship between the numbers to choose a method. For example, if students are finding the value of \(733-198\), they may choose to add on 2 to 198 and count on by place instead of decomposing a hundred and a ten.

• Create a set of cards from the blackline master so that each student will receive 1 card.

• Groups of 2
• Give each student a card.
• Give students access to base-ten blocks.

• “We are going to create our own subtraction expressions and find their values. You will walk around and find someone who has a different number than you. Create an expression and find the difference between your numbers. Show your thinking.”
• “Trade cards and find a new partner.”
• 12 minutes: partner work time
• Monitor for students who represent subtracting by place with:
  • a base-ten diagram
  • numbers, words, or equations

• Invite previously identified students to describe the steps they used to find the difference for one of their expressions.
• Consider asking:
  • “What questions do you have for _____ about their steps or their representation?”
  • “How did you use a similar representation? How were your steps the same? How were they different?”