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. Open number lines 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
- open number line

**Special materials needed:**

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