# 3. Subtract 10 and 100 mentally (FP)

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 mentally subtracting.

This lesson helps to build procedural skills with mentally subtracting by skip counting. Hundreds charts, counting strips, and open number lines are used here because they clearly show changes in numbers when subtracting by 100. This work develops students' understanding that we can use skip counting to add or subtract because when we count, we move along the number line.

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. Hundreds charts, counting strips, and open number lines are used to play a game. Students discuss the numeric pattern found in three-digit numbers when subtracting 100.

Key vocabulary:

• decompose
• decreases
• open number line

Special materials needed

• "How Close Can You Get?" Recording Sheet
• "How Close Can You Get?" Subtraction Cards
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