Lesson plan

# 5. Count back by tens and ones to solve two-digit problems (FP)

teaches Common Core State Standards MP1 http://corestandards.org/Math/Practice/MP1
teaches Common Core State Standards MP7 http://corestandards.org/Math/Practice/MP7
teaches Common Core State Standards 2.NBT.B.5 http://corestandards.org/Math/Content/2/NBT/B/5

# 5. Count back by tens and ones to solve two-digit problems (FP)

Lesson objective: Fluently use an open number line or hundreds chart to find differences by counting back or counting on.

This lesson helps to build procedural skills with whole number subtraction. Base ten blocks and hundreds charts are used here because they support counting back or on by tens. This work develops students' understanding that we can subtract numbers within 100 using our knowledge of place value.

Students engage in Mathematical Practice 7 as they look for and make use of structure, which is central to the work in this task. Students should identify the structure of place value in order to add or subtract within 100 as they use models, such as number lines and hundreds charts, to solve problems.

Key vocabulary:

• count back, count on
• difference
• hundreds chart
• base ten blocks

​​​​​​​Special materials needed:

• printable hundreds charts available
• place value tools (rods and units)