# 1. Add and subtract tens and hundreds mentally (C)

teaches Common Core State Standards CCSS.Math.Content.2.NBT.B.5 http://corestandards.org/Math/Content/2/NBT/B/5
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: Extend understanding that skip counting by tens and hundreds is critical to mentally adding and subtracting three-digit numbers.

Students bring prior knowledge of understanding place value in order to add and subtract from 1.NBT.C.5. This prior knowledge is extended to mental addition and subtraction as students develop mental strategies to find the sum or difference. A conceptual challenge students may encounter are misconceptions about the value of digits in numbers. For example, students may think that the 4 in 46 represents 4, not 40 or 4 tens.

The concept is developed through work with hundreds charts, counting strips, and number lines, which support students as they develop their skill to mentally add and subtract.

This work helps students deepen their understanding of number because skip-counting supports mental math and helps students develop an understanding of the numeric patterns in math. The number changes in the hundreds place when skip counting by hundreds. Numerical patterns also help us develop algebraic reasoning.

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 to mentally add or subtract numbers using numerical reasoning.

Key vocabulary:

• counting strip
• hundreds chart
• number line
• represent

Special materials needed:

• counting strips
• hundreds chart
• number line
• unifix cubes
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