# 9. Counting up and decomposing to solve a subtraction problem (FP)

teaches Common Core State Standards CCSS.Math.Content.3.NBT.A.2 http://corestandards.org/Math/Content/3/NBT/A/2
teaches Common Core State Standards CCSS.Math.Practice.MP8 http://corestandards.org/Math/Practice/MP8

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Lesson objective: Subtract within 1000 using decomposing and counting up to solve a subtraction problem.

This lesson helps to build procedural skill with whole number subtraction. A number line is used here because it supports the understanding that counting up by decomposed values can be used to solve a subtraction problem. This work develops students' understanding that decomposing a number can make it easier and more efficient to find a difference by counting up on a number line.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they decompose numbers using values that make counting up easier, while still maintaining oversight over the entire process of using counting up (the inverse operation, addition) to solve a subtraction problem efficiently and strategically.

Key vocabulary:

• counting up
• decomposing
• difference
• efficient
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