# 9. Fluently adding a two-digit number with a one-digit number (FP)

teaches Common Core State Standards CCSS.Math.Content.1.NBT.C.4 http://corestandards.org/Math/Content/1/NBT/C/4
teaches Common Core State Standards CCSS.Math.Practice.MP1 http://corestandards.org/Math/Practice/MP1

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Lesson objective: Add a two-digit number with a one-digit number, sometimes composing a ten.

This lesson helps to build fluency with addition. Cubes and written notation are used here to support students' need to concrete representation, but also support their understanding of decomposing and composing numbers in written form. This work develops students' understanding that when you have more than ten ones, you can compose a new ten.

Students engage in Mathematical Practice 1 (Make Sense of Problems and Persevere in Solving Them) as they practice combining two-digit numbers with one-digit numbers. In cases in which the numbers are reversed (i.e. 6 + 45 instead of 45 + 6), students may need to apply their knowledge of the commutative property.

Key vocabulary:

• compose
• decompose
• ones
• tens

Special materials needed:

• cubes (if needed)
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