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: 

• 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