Lesson objective: Solve elapsed time problems where the starting time is unknown by using a number line to represent subtraction.

This lesson provides an opportunity for students to apply their knowledge and understanding of telling time and number operations to solve a real-life situation. Students are asked to determine the range of starting times for making brownies given the elapsed times and the ending time. Students use a number line to understand the problem, the meaning of the question asked, and show their methods for solving the problem.

Key Concept students will use:

Skills students will use:

- a number line to show the time to the nearest minute and
- a number line to show how to subtract elapsed time.

Students engage in Mathematical Practice 1 (make sense of problems and persevere in solving them) as they use the number lines to show their methods for solving elapsed time problems.

**Key vocabulary: **

- elapsed time
- number line
- range of times

**Special materials needed:**

- Judy clock for demonstration
- Number line for demonstration
- Student clocks