Lesson objective: Solve word problems using a number line and check answers using another tool.

This lesson helps to build fluency with solving addition and subtraction word problems and using multiple tools to solve and check computation. Tape diagrams are used to help students interpret (decontextualize) the word problems. Number lines and ten frames are used here because they support students' understanding of addition as "putting together" and subtraction as "taking apart." This work develops students' understanding that the meaning of addition and subtraction remain the same, no matter which tool we are using.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) by noticing that the sum or difference remains the same regardless of the tool and/or strategy used. They may also articulate that they can use “adding on” to solve subtraction situations.

**Key vocabulary:**

- put together and take apart
- tape diagram
- number line
- unknown

**Special materials needed:**

- a variety of tools for students to use such as ten frames, arithmetic racks, number lines, snap cubes, or other concrete objects