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.


  1. 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.

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

    2. 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. ” (

  2. 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.

    1. this. any solution that relies on marking sand levels in an hourglass or measuring increments or intervals is wrong. the solution is simple and elegant.

  3. 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

  4. 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.

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

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

  7. 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.

  8. 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!).

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

  10. 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.

  11. 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.

  12. 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.


    1. I agree. All this obsession with how quickly it took is annoying, especially if I took considerably longer.

  13. 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.

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