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

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    1. 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.

    2. 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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