It’s the Friday Puzzle!


coverPlease do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.

I have 2 hourglasses.  One measures 7 minutes and the other measures 11 minutes.  What is the quickest way to use them to time the boiling of an egg for 15 minutes?
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle(UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

77 comments on “It’s the Friday Puzzle!

  1. Anonymous says:

    I’m scratching my head on this one. I haven’t got a clue.

  2. Carl Triggs says:

    About a minute to solve

  3. Chase says:

    Just to be cheeky, the “quickest” way would be 15 minutes. Though the “simplest” way took me about a minute to figure out. Nice puzzle.

  4. curtisfrye says:

    Less than a minute of twisting and turning.

  5. Martha says:

    Got it!

  6. geodetective says:

    found it. about 1 minute. Similar to the puzzle of 14 may 2010.

    By the way, I don’t like to boil eggs green… The hour glass of 7 minutes is enough.

    • Greyoak says:

      Wait what?
      Eggs aren’t supposed to be green?

    • geodetective says:

      When you boil eggs for 15 minutes or more, they yolk becomes green… Don’t boil them that long!

    • The Masked Twit says:

      Richard has not specified what type of egg it is.
      15 minutes might be the optimum time for boiling an ostrich egg.

    • Ken Haley says:

      The person might be at high altitude. On the topic of “Hard-boiled Eggs at Altitude” I found this quote from someone in Loveland, Colorado (altitude ~5,000 ft or 1500 m): “I’m in Loveland and I actually just use my electric kettle for small batches. I put 3-4 eggs in the kettle and set my timer for 15 minutes. When the timer goes off, they are just right. ” (

  7. elsiem says:

    About thirty seconds. I had the vague shape of the solution right away, but it took a little longer to nail it down confidently.

  8. vicinca says:

    About a minute. Nice one!

  9. Albrecht says:

    Do not assume that these hourglasses measure time linearly, as their speed actually depends on the amount of sand which remains in the upper part. They can only be used to measure the exact intervals they have been calibrated for. This rules out shorter solutions based on “partial times”; however, this approach provides an exact solution.

  10. Anders says:

    solved in about a minute (didn’t we have this one recently, or did I dream it?), and I agree the quickest way by definition has to be 15 minutes

  11. Anonymous says:

    10 secs to work out the answer.

  12. doug says:

    about a minute as well

  13. Almost too easy. And boiling an egg for 15 minutes will make it inedible.

  14. Lazy T says:

    Simple solution, very hard boiled egg
    Incidentally , on a food theme, the dara o’brien programme had a good puzzle this week can I suggest Prof W sets it for next friday and sends us all lots of dim sum in preparation?
    I like dim sum.

  15. Ed Wilson says:

    About ten seconds, then 30 to write it down, but I was assuming linear measurement (see Albrecht), so I may be wrong.

  16. Shoot the 7-minute timer first. Oh, hang on, that’s last week’s puzzle.

  17. Took about 1/7th of the first hourglass…

  18. Anonymous says:

    3 seconds, figured it out as I read it.

  19. D says:

    Nice puzzle. 2-ish minutes.

  20. Stan says:

    Nice to have a simple puzzle now and then. 5 sec.

  21. Berhard says:

    just a few seconds…

  22. About 6 seconds. Worked it out before finishing reading the question… Have the hard Friday puzzles all finished now? #scrapingthebarrell

  23. Anonymous says:

    Find someone with a watch and say “I’ll give you these two hourglasses if you’ll time my egg for me”.

  24. Duncan says:

    I’ve got two answers, I’m interested to see which one Richard thinks is right.

  25. AlienCG says:

    It took me about 2 minutes.

  26. RLQ says:

    About 15 seconds. This was one of the easier ones for me. I seem to get stumped on the ones no one else has problems with, lol.

  27. Anne Elk says:

    Couple of minutes to solve.

    (And if you heat the water from cold, straight from the tap, 15 minutes is a good time for three boiled eggs. And three minutes for toast soldiers!).

  28. Anonymous says:

    Took us 20 minutes…

  29. Matt says:

    Got it very quickly, within 20 seconds, seemed way too simple to me but I guess we’ll find out on monday.

  30. mightyhero says:

    Too easy.

  31. Jay says:

    I am 11 years old and figured it out in 10 seconds flat, using an hourglass to time those ten seconds.

  32. Less than ten seconds. Arithmetic is your friend.

  33. Alex says:

    Got it after about two seconds, a personal record. But I really don’t think the method I used can be the best.

  34. Tony says:

    30 secs unless Albrecht’s point matters

  35. Pat Nowak says:

    1. Turn over both hourglasses at the same time.
    2. Set your IPhone timer for 15 minutes.
    3. Stop cooking when the timer rings.
    4. Don’t allow yourself to be put to sleep watching the hourglass.

  36. John says:

    I got the answer in seconds.

  37. Jen says:

    A minute (but I was writing it down, I need pictures)

  38. Steve Augustin says:

    These people who said they got in a few seconds are probably not very aware of the passage of time. I suggest they actually time themselves for the real time they took to get it but not with an hour glass though.

  39. Wheee, first one in a while that I got fairly quickly. Say, was this inspired by the cicada thing making the rounds?

  40. The Pick Man says:

    Why would anyone want to boil an egg for 15 minutes?

  41. Fotoflex says:

    About a minute.

  42. Kat says:

    took me exactly 43 ms.
    Why is there not a single interesting comment among the first 53? All you guys always say is how long it presumable took you. Everything always has to be a contest for you. Let me assure you that you all win big time at being an annoying pricks.


  43. Chicken Lady says:

    What are toast soldiers? Anything like goldenrod eggs?

  44. Zoe Butler says:

    About one minute.

  45. John says:

    Why are the hour glasses 7 & 11 minutes, shouldn’t they be 60 minutes.

  46. Got it very quickly this week. But I agree with other commenters — who would want to boil and egg for 15 minutes? Is there a way of using the two hourglasses to time an egg for four minutes? Yes, there is.

  47. Anonymous says:

    Got the idea instantly, took about 30 seconds to work out the details.

  48. One Eyed Jack says:

    Got the puzzle, but who the hell boils an egg for 15 minutes?

  49. About two seconds. But why would it take 15 minutes to boil an egg? Is it an ostrich egg?

  50. Anonymous says:

    Very simple

  51. Mervulon says:

    It took me 22 minutes.

  52. Nicola says:

    a few seconds were enough…

  53. nainfiort says:

    it took me 15 minutes ^^

  54. TS says:

    It took me three and a half minutes to figure this one out, which is exactly the time I normally boil my eggs for.
    Now all I have to do is forget the solution so I can use the puzzle to time my boiling eggs.

  55. niko says:

    It took me 22 min to get the eggs on the table. And, yes, they were green.

  56. Jefferson says:

    I was able to the solve the problem am allergic to eggs. I think I need to go to the hospital now.

  57. dayuntoday says:

    less than a minute. Curious to see if I’m right.

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