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 reallife 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:

We can use representations such as a number line to solve situational problems involving addition and subtraction of time in minutes.
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