Students will be able to solve elapsed time situations using a number line as a tool.

Telling time is not enough. Students need to understand and apply strategies to solve elapsed time situations that are real world. This knowledge is essential in creating a deep understanding of using clocks as a tool to measure the passing of time.

20 minutes

To review the concept of telling time, and warm-up the students, I project an interactive clock. I display times and have the students write that time on their white boards. If you can't get to the site, you can always use a Judy Clock, your own classroom clock, or clock pictures.

I then explain an elapsed time situation to solve. For example, if the clock reads 1:47, I might say, "In 3 hours and 15 minutes I will start making my dinner. What time will it be?"

I watch for strategies as the students work. I am hoping to see some addition problems and use of a personal clock. I am confident many will not use a number line, which is what I will introduce today.

After sharing some of the student strategies, I put an open number line on the board and reference looking at the time as an expanded form problem, or as a problem worthy of using a distributive type solution. The number line can help us keep track when we do that (MP5 - Use appropriate tools strategically).

This clip is a wonderful example of students (not mine!) teaching us how to use a number line to solve elapsed time. You may want to watch it for your own tutorial, or show it to your class. If you use it with your class, remember the importance of pausing/stopping and talking (or giving students time to read problem, or catch up with their own thinking). Giving students a little whiteboard to follow along, and watching it twice, might be a good strategy to use as well.

Remember, the kids always say it best! I think the best thing to do when teaching the use of an open line and time is to let the kids make friendly numbers, or to make the jumps that make sense to them. This builds their mental math power and the line is just a tool to help them keep track.

30 minutes

For practice, I continue stating elapsed time situations on the board and have students solve on their white boards. As they work, I circulate the room and prompt where needed. I also ask students to share their strategies with their shoulder partners.

We also share as a whole class by presenting the number lines. During this time, I ask the students to verbalize to me what to do on the board. In doing this, I push them to communicate their thinking more precisely (MP3, MP6), rather than drawing their strategy on the board without having to verbalize the process.

I also ask a few to come up and show their different strategies for the same situation.

This student begins by counting by 10's and then adding the one minute intervals (MP8 - Look for and express regularity in repeated reasoning).

This young man approached the problem by adding one minute intervals first, then 5 minute intervals, then the 10's, and finally by 5's and 1's again (MP7 - Look for and make use of structure). He was creating "friendly" or benchmark numbers that made sense to him mentally.

10 minutes

For closing, we review that elapsed time is a way of measuring the passing of time from one event to another. Knowing how to do this can help us manage our time efficiently and get places on time!

For home practice, I ask the students to keep a chart of activities from the time they get home until the time they go to bed. They must write down the start and end times for their evening. We will use these situations in our warm up activities in future lessons.

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