# 5. Use number lines to find differences between lengths (FP)

teaches Common Core State Standards CCSS.Math.Content.2.MD.B.5 http://corestandards.org/Math/Content/2/MD/B/5
teaches Common Core State Standards CCSS.Math.Practice.MP6 http://corestandards.org/Math/Practice/MP6
teaches Common Core State Standards CCSS.Math.Practice.MP7 http://corestandards.org/Math/Practice/MP7

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Lesson objective: Find the difference between lengths using a number line.

This lesson helps to build procedural skill with whole number subtraction. A number line diagram is used here because it highlights part-whole relationships and supports the strategy of "counting on" to find a missing addend or "counting back" to subtract. This work develops students' understanding that we can use a number line to solve many types of addition and subtraction problems related to length.

Students engage in Mathematical Practice 6 (Attend to precision) as they label number line diagrams precisely. They also engage in Mathematical Practice 7 (Look for and make use of structure) as they look for number patterns and relationships on the number lines to develop computational strategies.

Key vocabulary:

• count back
• count on
• difference
• number line diagram

Special materials needed:

• blank number lines (optional)
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