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Two sand glasses – Brain Teaser

hour_glass_1What is the quickest way to time a 9 minute cooking session with two sand-glasses of 4 and 7 minutes respectively?

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A riddle to challenge your mind | Back – Next | Arrange the numbers 1-9

Posted by on May 17th, 2009 and filed under Discussions. You can follow any responses to this entry through the RSS 2.0. You can leave a response by filling following comment form or trackback to this entry from your site

9 Responses for “Two sand glasses – Brain Teaser”

  1. Mr. W says:

    Tell your cook to do it :)

  2. Muhammad says:

    At Time=0
    Start both 4 & 7 Timer
    At Time= 4
    Restart The (4 min.) Timer
    At Time = 7
    Restart The (7 Min.) Timer
    ** Begin The Cooking
    At Time = 8
    (When The 7 Min. timer run out)
    restart the 4 min. timer
    At time= 12
    restart the 4 min. timer

    So : the cooking time = 1 + 4+ 4 = 9 min.

  3. admin says:

    Good job everyone,
    Here is my solution. It is fairly similar to some of yours.
    Start both timers simultaneously, then turn the 4 minute one over as soon as it runs out. Then when the 7 minute timer runs out, turn it over. When the 4 minute timer runs out for the second time, turn over the 7 minute timer which has been running for 1 minute, and when it runs out, hey presto 9 minutes have elapsed.

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