Teacher Tool of the Month: Using Landmark Numbers to Add on the Open Number Line
Written By Thera Pearce in Math Learning on December 19th, 2012
Each month we will be featuring one of our FREE Teacher Tools for interactive whiteboards. Ready for use in elementary classrooms with any type of interactive whiteboard, these high-quality virtual manipulative tools help teachers connect students with great ways to make sense of math. Use these tools to create an environment for students to explain, discuss, and defend their mathematical thinking.
This month we are featuring “Using Landmark Numbers to Add on the Open Number Line ” for Grade 3.
This lesson describes an open number line virtual manipulative for teachers to use in a variety of instructional contexts. In contrast to number lines where all of the counting numbers are visible, the open number line displays only the starting points, change amounts, and resulting values. Using the open number line model, students leverage landmark numbers such as multiples of tens or hundreds to solve addition problems.
Sample Lesson |
Objective: |
Students use landmark numbers as a mental math strategy when adding. |
Background: |
With a problem such as 96 + 79, we want students to realize how close 96 is to 100. By decomposing 79 into 4 + 75, students can add 96 + 4 to make use of 100 as a landmark number. |
Instruction: |
1. Write 38 + 6 on the board. Ask, “Which one of these two values is close to a landmark number, and what is that friendly number?” Call on a student to respond. Ask, “If we use 40 as a landmark number, what new equations could we generate to find the sum of 38 + 6? Discuss your strategy with your partner.” After students discuss, call on an individual to respond. As the student responds, write the new equations on the board and ask the student to draw the strategy as an open number line. Sample student responses and explanations: 2. Continue the conversation with additional equations such as 47 + 24 and 28 + 46. 3. Show the DreamBox open number line on the interactive white board. (Example: 96 + 79) Ask, “How can we use a landmark number to solve this problem? Discuss your strategy with your partner and draw your own open number line.” 4. After students have developed and discussed their strategies, ask for a volunteer to type the number on the line that will become a landmark to use. That student types the number in the open number line. Ask, “Why is that number close to a landmark number?” Sample student responses: 5. Ask another student, “How far do we need to jump to reach the landmark number?” Request this student to model that jump on the white board as well as type in the next point on the number line and explain his direction and strategy. Sample student responses: 6. After the student has made the jump ask, “What is the next jump that we will make as well as the next location on the number line?” Invite individual students to respond and then ask them to describe how they arrived at an answer. Discuss their strategies. Sample student responses: 7. Click the “Next” button on the DreamBox open number line tool and begin a new problem. Engage all students in using the open number line to solve the problem and ask them to explain their thinking and justify their answers. Use the questions above when appropriate to scaffold as needed. |
Common Core State Standard |
Grade | CCSS ID | Domain | Cluster | Standard |
3 | 3.NBT.5 | Number and Operations in Base Ten |
Use place value understanding and properties of operations to perform multi-digit arithmetic. |
Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction. |
3 | 3.SMP.5 | Standards for Mathematical Practice |
Use appropriate tools strategically. |
Try the lesson in your classroom today!
Thera Pearce
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