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Safe Cracking

by Dan Meyer


Act One

  • Dan Meyer

    Dan Meyer

    September 30, 2013

    How long would it take to do what he did?

  • 1.

    What's a number you know is too high?

  • 2.

    What's a number you know is too low?

Act Two

  • 3.

    What information is important here?

  • Teacher note
    It seems like there are three important quantities here:

    -- How long it takes him to write down one four-number combo.

    -- How long it takes him to test one four-number combo.

    -- How long it takes him to cross off one four-number combo.

    You can have students collect their own data by timing the video and by seeing how fast it takes them to write down / cross off a few four-digit numbers.

    Then you need to know what the final combination is. A close look at the video reveals "8020," but feel free to let your students make that determination.

Act Three

  • Teacher note
    So he writes down numbers 0000-9999. (Seems like a waste of time. What if the combo was "0000?")

    He tests out numbers 0000-8020.

    I assume:

    -- Write a four-digit number down: 1 second.

    -- Try it out and cross it off: 4 second.

    Total time to write them down: 1 sec/number * (9999 - 0 + 1) numbers = 10,000 seconds.

    Total time to try them out and cross them off: 4 sec/number * (8020 - 0 + 1) numbers = 32,084 seconds.

    Total is 42,084 seconds or 701 minutes or 11.68 hours, which is a lot less time than I would have thought. Assuming he could do that for a couple hours per night without going crazy, and he'd be done in a week!


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