Lesson objective: Understand what it means to compare known numbers to find the unknown.

Students bring prior knowledge of comparing numbers from 1.NBT.3. This prior knowledge is extended to explaining what comparison situations mean as students explore the relationship between addition and subtraction. A conceptual challenge students may encounter is understanding the comparison language.

The concept is developed through work with number lines, which allow students to compare two-digit numbers and see the distance between these numbers using a visual model.

This work helps students deepen their understanding of numbers and operations because by determining the distance between two numbers, students understand how these numbers are related.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they notice that they can use the properties of operations and the relationship between addition and subtraction to solve comparison situations using the number line. Students also engage in Mathematical Practice 7 (Look for and make use of structure) as they utilize their knowledge of number sequence when comparing two numbers using “more” and “less" than.

**Key vocabulary:**

- addend
- addition
- between
- compare
- comparison
- comparison statement
- distance
- equation
- greater than
- less
- less than
- missing number
- more
- number line
- sequence
- subtraction
- tape diagram
- two-digit number
- unknown number
- visual model

**Special materials needed:**

- white boards/markers or paper/pencil
- open number lines