Lesson objective: Understand that number lines can represent the process for solving addition and subtraction elapsed time problems.

Students bring prior knowledge of telling time to the nearest five minutes and representing time on a number line; (Grade 2, Unit 4; 2.MD.7, 2.NBT.2) . This prior knowledge is extended to telling time to the exact minute as students use the number line to solve addition and subtraction elapsed time problems. A conceptual challenge students may encounter is understanding the meaning of the unknown in elapsed time problems. Another conceptual challenge students may encounter is understanding that sixty minutes is equal to one hour and that elapsed time problems can involve more than one hour.

The concept is developed through work with a number line, which represents the students' reasoning about the problem structure, the meaning of the question, and their solution strategy.

This work helps students deepen their understanding of operations because students apply their addition and subtraction to solve problems involving elapsed time. Students use representations and multiple strategies to solve for the unknowns such as: if the starting and ending times are known, they can find how much time has elapsed by using a number line or clock face and counting on or back using benchmark numbers (adding by ones to five or ten and using fifteen or thirty).

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they represent their understanding of the word problem structure and the meaning of the problem question.

**Key vocabulary:**

- elapsed time
- number line

**Special materials needed:**

- Judy clock for demonstration and student clocks
- minute number line for demonstration