Lesson objective: Relate skip counting to mentally subtracting.

This lesson helps to build fluency using mental math to subtract by skip counting backwards. Hundreds charts and base ten blocks are used here because they clearly show changes in numbers when subtracting one hundred from a value. This work develops students' understanding that we can use skip counting to add or subtract because when we count, the values change following a predictable numeric pattern.

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 and base ten blocks can be used while students play a game. When students no longer need these tools, they will show their proficiency with the concepts by using mental math to subtract one hundred. 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