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        Calculating Elapsed Time

        Students practice the concept of time passage by counting, subtracting and regrouping in base 60.

        Lesson Summary


        Students learn to figure out elapsed time by counting, subtracting and regrouping in base 60. This activity is motivated by a Cyberchase episode in which Digit and Hacker compete in a cooking contest.

        Grade Level:


        Suggested Time

        60 minutes

        Media Resources

        Cooking Contest QuickTime Video
        Figuring Out Elapsed Time QuickTime Video


        Time Passage handout
        Assessment: Level A
        Assessment: Level B
        Answer Key

        The Lesson

        Part I: Learning Activity

        Note: Elapsed time is treated in this activity as an application of addition and subtraction of two- and three-digit numbers. When the elapsed time crosses an hour boundary, either addition of time (to identify a later time) or subtraction (to identify an earlier time or elapsed time) becomes an example of regrouping three-digit numbers, but in base 60 instead of base 10.

        1. Read students the following: "Digit and Hacker are in a cook-off for the Golden Hat. They must cook three dishes in three different locations within certain time limits. Digit's timer is stolen, so he has to use a watch to time his cooking."

        2. Read this warm-up exercise: "Digit's first dish needs 5 minutes to cook, and the current time is 9:14. At what time should Digit remove his dish from the oven?"

        3. Discuss the students' answers, and talk about counting. For instance, 14, 15, 16, 17, 18, 19, and emphasize that you do not count the first number, 14, as one of the passing minutes. Students can create their own timelines, or you can use one for the classroom. Or discuss adding 14 + 5 = 19 as a second approach to the time problem. This is an important developmental step.

        4. Tell the students that they will watch a video segment in which Digit must send in his cooking dish to the judges by 9:43, and it's currently 9:27. His dish takes 15 minutes to cook. Will he have enough time for his dish to finish cooking? Give them time to figure out their answers, and watch for how they use these time points' relationships to 9:30 and 9:45.

        5. Show the Cooking Contest QuickTime Video.

        6. Distribute the Time Passage handout, and ask the students to answer Part I.

        7. Read the following: "The last dish is a CyberSouffle, a very light and puffy baked egg dish. It must cook for precisely 12 minutes. If Digit puts it in the oven at 9:50, when should he remove it?" Give the students time to figure out their own answers.

        8. Watch the Figuring Out Elapsed Time QuickTime Video.

        9. Ask the students to answer Part II of the handout.

        Part II: Assessment

        Assessment: Level A (proficiency): Students are asked to calculate a new time from a given time. Some of the times cross the hour.

        Assessment: Level B (above proficiency): Students are given the context of the starting times of events at a track and field meet and are asked to figure out the elapsed time between various pairs of events.


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