# 2. Subtract 10 and 100 from various numbers mentally (C)

teaches Common Core State Standards CCSS.Math.Content.2.NBT.A.2 http://corestandards.org/Math/Content/2/NBT/A/2
teaches Common Core State Standards CCSS.Math.Content.2.NBT.B.8 http://corestandards.org/Math/Content/2/NBT/B/8
teaches Common Core State Standards CCSS.Math.Practice.MP8 http://corestandards.org/Math/Practice/MP8

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Lesson objective: Relate skip counting to mental addition and subtraction.

Students bring prior knowledge of adding multiples of ten from 1.NBT.C.5. This prior knowledge is extended to mentally adding and subtracting by 100 as students work on a task subtracting 100 from 993. A conceptual challenge students may encounter is a weak understanding of place value. For example, students may think that the 4 in 46 represents 4, not 40 or 4 tens.

The concept is developed through work with ten frames, counting strips, or number lines, which help students see the place value in a number.

This work helps students deepen their understanding of operations because students compose numbers to 1000 by skip counting from any number by 10s or 100s. The ability to add or subtract, from any number, is foundational for mental math skills. Exploring the patterns found when skip counting helps students mentally compute fluently and efficiently.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they notice the repeated pattern when skip counting and how this helps them mentally add or subtract numbers using numerical reasoning. This repeated pattern will be highlighted as students work with hundreds charts, counting strips, and open number lines.

Key vocabulary:

• backwards
• counting strip
• decompose
• decreases
• open number line

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