Lesson objective: Relate skip counting to mental subtraction.

Students bring prior knowledge of adding and subtracting multiples of ten from 1.NBT.C.5. This prior knowledge is extended to mentally adding and subtracting by 100 as students work on a task subtracting 100 from 993. A conceptual challenge students may encounter is a lack of place value understanding. For example, students may think that the 4 in 46 represents four, not 40 or four tens.

The concept is developed through work with hundreds charts which help students see the number numeric patterns.

This work helps students deepen their understanding of operations because students compose numbers to 1000 by skip counting from any number by tens or hundreds. The ability to add or subtract, from any number, is foundational for mental math skills. Exploring the patterns found when skip counting helps students use mental math to compute fluently.

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. This repeated pattern will be highlighted as students work with hundreds charts, counting strips, and open number lines.

**Key vocabulary:**

- backwards
- decompose
- decreases
- hundred chart
- sequence
- value