It’s the Friday Puzzle!



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

Imagine a water-cask and three different sized water-taps.  The smallest tap can fill the water-cask in 20 minutes.  The middle tap can fill the cask in 12 minutes. And the largest tap can fill the water-cask in 5 minutes. How long does it take to fill the water-cask with the three taps together?
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.

70 comments on “It’s the Friday Puzzle!

  1. Anonymous says:

    Done, now I’m off to boil an egg…

  2. eyeonicr says:

    About 30 seconds, but I had to use a computer to work with the fractions (so add about half an hour and a wrong answer).

  3. Roland says:

    Took me about 15 seconds to type in the equation.

  4. Anonymous says:

    It takes longer to fill the cask than to find the answer.

  5. Andrew says:

    3 divisions, one addition and a further division took about 30 seconds.

  6. Eddie says:

    Easy but still good.

  7. Bryan Atherton says:

    Will confirm on Monday if I’ve got the right answer.

  8. Got it. This one was easy. Much easier than finding three mismatched taps close enough together fill the same cask at one time…

  9. Numbers guy says:

    It took about a third as much time as the answer. Good question. I will pass this on to my daughter who’s just started GCSE maths.

  10. The Masked Twit says:

    It took me about one ninetieth of the of the time it would take all three taps to fill the cask to solve the puzzle.

  11. Pitmonster says:

    Less than a minute.

    That’s how long it took me, not the answer.

    But I had to use Excel…

    • Yeah – did the calculations in Excel two – chosen number of minutes puzzled me until I had the solution. They were chosen to give a nice answer to the puzzel.

    • mittfh says:

      About a minute in Excel, plugging in a value of 100 units for the volume of the cask. As others have said, the different flow rates were deliberately chosen to ensure a nice round number when combined.

  12. Miss Chili says:

    Undoubtedly, I’d drop the d@mn cask in the middle of it all and ask myself why I’d bothered hunting down the three mismatched taps… 😀

  13. Pat Harkin says:

    Two hours and still filling. Perhaps I should close the stopcock at the bottom of the cask and try again.

  14. Andy Jackson says:

    About a minute – now I’d better turn those taps off!!

  15. Anonymous says:

    Less than a minute to solve.

  16. ChrisR says:

    took me a few (<5) minutes – from first principles (let 'x' be the amount of water in the tank etc), checking whether the formulae I'd got actually made sense (that took the most time!), and then confirming it by imagining x was really 100 litres.

    • ChrisR says:

      but then, as Paddington would say, it would take no time at all because it’s still full up from before

  17. Nice question, but solved in about 30 seconds once I had some paper and remembered how adding fractions works!

  18. Anonymous says:

    Good thing I worked it out before I looked at the comments. Yet again it seems some people think it’s appropriate to give the answers away and spoil it for others. There shouldn’t be a comments option on the question page, there should only be one on the Monday morning answer page.

  19. Karleb says:

    20 seconds to figure out what to do another minute or so to do it in my head

  20. DiscoveredJoys says:

    A minute, using paper and Post-it to keep track.

  21. Stewart says:

    Five minutes to solve it. Had to remember my fractions from school days.

  22. Gema says:

    It takes nothing because we are talking about a WATER-cask (that’s the clue).

  23. Dharmaruci says:

    mr wiseman’s puzzles are unfair to those of us who canno’t solve them. i must post only how long soln took. but this one o canno’t solve so my posting voice is silence.

    unfiar, mr wiseman!

    • Dave says:

      Obviously you need more practice at being silent. Tell you what, see how long you can manage from now on. We’re all rooting for you.

  24. Alma says:

    WHY do people continue to insist on posting their answers – or obvious clues to the answers?? It’s fun to read people’s comments and see what they think of these puzzles – but don’t they understand that it spoils it for others just because they have a need to show off?? And don’t they understand the English that Richard uses to ask them NOT to post their answers?????

    • Mr Self Righteous Twat says:

      I think that they post their answers just to wind up people like you and me Alma.
      Works every time too.

  25. Julia says:

    so glad I didn’t read the comments today!

    I enjoyed this puzzle. I needed pen and paper and had to imagine that it’s a 60-litre cask. Took me about two or three minutes to solve.

  26. @Sceptic_Tank says:

    It took me less time than the answer, longhand, as taught for the eleven-plus maths exam back in the 1950s. Nostalgia isn’t what it used to be.

    Totally agree with comments re people posting the answer. I never look at the comments before solving the puzzle for myself (or giving up, which I did last week!).

  27. Anonymous says:

    Did it the old fashioned way with paper and pen. Most of the time taken up with finding the pen.

  28. Lazy T says:

    Fractions revisited, an answer appeared on the back of my old (and favorite) envelope in about 1/9 of the time it takes to fill the vessel.

  29. less time to solve it than to type this message saying that I have.

  30. JJM says:

    Couple of minutes. Somewhat ruined by reading comments before attempting (rookie error as ever – but why do people insist on posting answer or “cryptic suggestions”, enough of the willy-waving). But reached an neat answer on a post-it note, no excel or anything needed.

  31. 2-3 min. I guessed the answer and checked with my rusty maths.

  32. Stan says:

    It took just a few seconds to make a good guess.. The way I did it was that I figured it had to be less than 5 min and that made me think about a formula for adding resistors. That gave the same answer as the others who sadly gave away the answer. My way wasn’t as good as theirs that was more principled than my way which was just a guess. But it’s interesting that the principle is actually the same. So now I have deeper insight into that formula for adding resistors.

  33. Ken Haley says:

    Solution in a few seconds: 11 keystrokes on an RPN calculator. (I love RPN!)
    Of course, I’ve seen this type of puzzle before.

  34. Karen says:

    I was thinking that finally there was a reason to have studied calculus (which I don’t remember because I never use it,) but you all only used fractions so even though I didn’t get the hints, I have no excuse not to try it. I always use the trial and error method.

  35. Anonymous says:


  36. mightyhero says:

    Too easy.

  37. Jeff Scanlan says:

    I think I got it but will wait ’til Monday to find out the answer. If correct, than it took me a bit longer to figure answer out then it would be to fill the casket. 🙂

  38. took about a minute less than to solve it

  39. John says:

    Have we had the Problem versus Puzzle debate lately ?

  40. pete says:

    didn’t use Excel (ok open office spreadsheet), less than 30 seconds. Now if it were full of beer…..

  41. Gene says:

    I can’t post how long it took because it’s the same as the answer. 😦

  42. One Eyed Jack says:


  43. Merari says:

    Took me 20 seconds in my head.

  44. ubi dubium says:

    About 30 seconds, including finding a pencil to work the fractions. I used to be able to do this stuff in my head.

  45. Chakolate says:

    About 15 seconds, but I did it in my head.

  46. issahamati says:

    About 30 seconds, but I did it in paper

  47. jean philippe says:

    yeah! I guessed the first comment right!

    1) I read and work out the puzzle
    2) According to the answer, I try to guess the first spoilery comment (the very first one actually)
    3) THEN, I read the comments…

    Got it right this time!

  48. ormaza says:

    Three minutes

    Saludos. Iñigo👀🇪🇸

    El 06/09/2013, a las 09:31, Richard Wiseman escribió:

    > >

  49. wallyadmin says:

    It took me the same amount of time to arrive at the solution as it takes the three taps to fill the cask.

  50. Mike Torr says:

    Took me about 15 seconds, mainly because I could see what was going on so I started calculating as I read through the question. I must have swallowed an extra maths pill tonight…

  51. Anonymous says:

    15 secs

  52. ART WEBB says:


  53. dayuntoday says:

    It took me almost exactly as long to find the answer as it does to fill the cask! 🙂

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