Lesson objective: Add and subtract to solve elapsed time problems using a number line to show mathematical reasoning.

This lesson helps to build procedural skill with solving for the unknown in time problems. A number line is used here because it is a useful way to represent computation strategies. This work develops students' understanding that a number line can be used to solve elapsed time problems.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they use the number line to understand the problem structure and what needs to be solved for the problem. The number line is used to represent the students' strategies for the computation.

Students engage in Mathematical Practice 4 (Model with mathematics) as they use a number line as well as addition and subtraction equations to represent time problems. The students will observe the inverse relationship between the addition and subtraction operations.

**Key vocabulary:**

- elapsed time
- number equation
- number line

**Special materials needed:**

- clock for each student
- Judy clock for demonstration
- number line for demonstration