# 8. Subtracting multiples of ten (C)

teaches Common Core State Standards CCSS.Math.Content.1.NBT.C.6 http://corestandards.org/Math/Content/1/NBT/C/6
teaches Common Core State Standards CCSS.Math.Practice.MP6 http://corestandards.org/Math/Practice/MP6

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Lesson objective: Understand that they can subtract groups of ten

Students bring prior knowledge that it is more efficient to add by a group of ten instead of ten ones from Grade 1, Unit 11 (1.NBT.C.4). This prior knowledge is extended to include subtraction as students solve subtraction problems involving multiples of ten.  A conceptual challenge students may encounter is subtracting by groups instead of by single units.

The concept is developed through work with a strip of arcade tickets shaped like a base ten rod, which helps students to subtract using groups of ten.

This work helps students deepen their understanding of equivalence because ten ones is the same as one group of ten. Students can subtract using groups of ten instead of taking away ten ones.

Students engage in Mathematical Practice 6 (Attend to Precision) as they determine leftover amounts of tickets and explain their reasoning.

Key vocabulary:

• subtract
• take away
• tens

Special materials needed:

• base ten blocks (optional)
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