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Eggalicious (Posted on 2002-07-05) Difficulty: 2 of 5
You have to boil a single egg for exactly nine minutes.

All you have are two hourglasses, able to measure (in sand) seven and four minutes, respectively.

How quickly, using only these two hourglasses, can you boil an egg?

  Submitted by qball    
Rating: 2.7857 (14 votes)
Solution: (Hide)
The trick to solving this problem is realising that you are dealing with hourglasses which can be reversed.

The following solution would not be possible with other time measuring devices:

  • Time = 0: Set T7 and T4, start cooking the egg.
  • Time = 4: T7 has 3 left, re-set T4, egg is 4 min cooked
  • Time = 7: Re-set T7, T4 has 1 left, egg is 7 min cooked
  • Time = 8: T4 runs out; T7 has 6 left. Rotate it. It now has 1 left. Egg is 8 min cooked
  • Time = 9: T7 runs out, egg is fully cooked.
  • Comments: ( You must be logged in to post comments.)
      Subject Author Date
    SolutionSolutionK Sengupta2007-05-28 13:43:25
    Solutionyou're all wrong...Steven Lu2004-12-30 16:08:39
    The correct solution (simple and elegant)kowalski2004-08-08 20:54:49
    The correct solutionKeith Jones2002-12-10 10:32:16
    SOLUTION FOR EGGALICIOUSchristopher hii2002-07-12 03:18:52
    re: 16 minute queryTomM2002-07-11 12:01:34
    16 minute queryDulanjana2002-07-11 04:33:35
    re(3): Solution:TomM2002-07-09 08:34:51
    re(2): Solution:levik2002-07-09 08:06:00
    re: Solution:G. Steve Arnold2002-07-09 07:53:36
    SolutionTwo SolutionsYour Mom2002-07-09 07:52:22
    SolutionSolution:G. Steve Arnold2002-07-09 07:44:56
    re(2): try..Rupert2002-07-09 07:03:09
    Some ThoughtsWhoopsDulanjana2002-07-08 06:02:24
    SolutionSolutionDulanjana2002-07-08 05:59:18
    READ this not the one belowqball2002-07-06 23:23:32
    jeremyqball2002-07-06 23:22:34
    No Subjectjeremy2002-07-06 20:28:43
    Solutionre: try..TomM2002-07-05 12:51:08
    Some Thoughtstry..Cheradenine2002-07-05 07:18:06
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