It’s the Friday puzzle!


petri_dish_13_october_2005Well, what a week.  I am in NYC now for a few meetings and a skeptics conference.

Thanks to everyone who contributed to the confession post.  It received an amazing response, and feel free to continue posting your confessions and helping others.

So, to the Friday puzzle….

A Petri dish contains a colony of bacteria. Every minute every bacterium divides into two. The colony was started by just one cell at noon. 47 minutes later the Petri dish was half full. At what time will the dish be full of bacteria?

The answer to this puzzle, and 100 others,  can be found in a new kindle ebook called PUZZLED, and is available in the UK here and USA here.


88 comments on “It’s the Friday puzzle!

  1. TS says:

    Ah, the old “scum in the pond” riddle.

    Took me zero time to solve.

    If you keep this up I will gain confidence and start to feel intili.. intiegen…. clever!

  2. Jason says:

    Toooooo easy.

  3. horuskol says:

    1 second to come up with the wrong answer… another second to realise I was dumb and come up with the right answer 🙂

  4. Colin Coghill says:

    Pretty sure the obvious answer’s not the correct one, but without pencil
    and paper handy I’m having trouble thinking it through.

    • FrankN.Stein says:

      you’re thinking too complicated! this is only a matter of understanding the question, there’s no need for writing anything down.

  5. lakritze says:

    Knew that one. Nice pic of the petri dish, though. 🙂

  6. Lucia says:

    it take few seconds, maybe ‘couse I know it! 😀

  7. Henk says:

    0s, way too easy

  8. taodih says:

    old riddle but fun

  9. nikki says:

    Took me a minute to find the time again to say what time it would be full. I like ones I can do quickly, makes me feel brainy for the day!

  10. Jareth says:

    About as long as it took to finish reading it. But I cheat by being a physicist who’s used to this sort of problem (though not example)

  11. henrik says:

    Didn´t know it. Took a sec or two, thats all. Easy yes.

  12. Bjarte says:

    If I’ve got the correct answer, it took me about 5 seconds.

  13. shpalman says:

    I already knew this one.

  14. Handysmerf says:

    This answer took me exactly 1 minute !

  15. scibuff says:

    waaaay toooooo eeeaaaassssyyyy

  16. ButMadNNW says:

    Maybe 20-30 seconds given that I briefly got sidetracked by over-thinking.

  17. rhabarber says:

    I am sure that the ‘right’ answer will actually be wrong, because it would only be correct in a liquid culture of bacteria, but not on a Petri dish. On a Petri dish there are space limitations – only bacteria with room about them can grow. Especially on a dish started with only one colony that is now half full, most of the bacteria are stuck in a bulk of bacteria and can’t grow and divide! And by the way, a division time of 1 minute is really unrealistically short.

    Sorry for this pedantic post, but I’m working with E. coli bacteria.

    • Alison says:

      Even in liquid culture it depends how good your shaker is… Not to mention the fact that though your bacteria *should* grow so fast, they are bastard bacteria who want to foil your experiment and make you stay in the lab late because you can’t induce them until you have X amount… I am sure the bacteria somehow know this!

      As for the puzzle, as long as it took me to read it.

  18. Martin Lascelles says:

    If it really took 47 minutes to get half full, dividing every minute, it has to be one hell of a big petri dish, more like a petri swimming pool!

  19. Melanie says:

    LOL, rhabarber. I was about to post just about exactly what you did, from space limitations to the division time. I spent years working with E.coli.

  20. Nick Sharratt says:

    Knew it, so knew the answer before finishing reading. (although the puzzle over simplifies reality but we can ignore that)

  21. grthink says:

    About 30seconds of rereading the question and I go tit. Either I’m getting more clever or the questions are getting easier.

  22. uksceptic says:

    Think I got this one.

  23. Timo says:

    its like that question: Marys father has five daughters, Lala, Lele, Lili, Lolo and …?
    to easy.

  24. Bucket says:

    Yay! Got it in less time than it took to read the question. I’ve seen variations of this one before though.

  25. Carl says:

    Knew it instantly. The way I remember it presented when I was a kid was slightly different to this, which made it just a tiny bit less obvious. I’ll give it on Monday, cos it kind of gives the answer away if you’ve heard both versions.

  26. Simply, it takes me a few seconds.

  27. CubanMule says:

    What kind of bacteria?

  28. Chris says:

    I really used a calculator and now, after finding the answer, I’m banging my head on the table.
    What a simple one!

  29. stunt_girl says:

    D’oh! Came up with the wrong answer in 0 seconds (thinking stupidly along the lines of shooting arrows at tortoises), then used some basic maths to get the right answer in about 15 seconds, before realising that once again there is a logical shortcut!

  30. mylittlestitches says:

    I like to think I have a ridiculously illogical brain and can never answer these things. I feel a bit upset this one was so easy. But also quite smug too.

  31. Jonathan Swift says:

    Do any of the bacteria die?

  32. Gordon says:

    immediately solved

  33. ScreamingGreenConure says:

    I know this one!!!!! I am so excited! I might fall down!

  34. I remember this being on quiz show when I was a kid (Ask The Family?)

  35. icecreamqueen says:

    easy peasy lemon squeezy!

  36. arthwollipot says:

    Ha. Once again, I got it instantly.

  37. Sidebernie says:

    Unless there’s something _really_ sneaky going on, the answer was obvious right away. But I’ve heard problems like this before, which probably helped.

  38. joebennett says:

    Got it straight away. But your I0 I0 II = I0.50 (from the ’59 Seconds’ book) had me going for more than an hour – at which point I gave up. And I didn’t get the teacup either.

  39. Cosmin Romania says:

    0.01 seconds.

  40. FrankN.Stein says:

    it is easy when you are in the right mindset, but it’s a nice test if you get easily distracted by somebody making a big show or if you can focus on the basic points of a problem.

  41. Anonymous says:

    @Chris: the same hear. ^2log2.8*e10=….. -47 = ….
    The answer made me smile.

  42. Oase says:

    @Chris: the same here: ^2log2.8E14 = … -47 = ….
    The answer made me smile!

  43. lflcorreia says:

    Easy! Get it immediately. Forget the complicated calculations. The key sentence is: “every minute every bacterium divides into two” – this is what is called exponential growth.
    There’s only one problem: we must assume that not a single bacterium dies before replicating itself – that’s not what happens in nature.

  44. This might be more deceptive, at least to the visual thinkers, if you displayed the problem along with a linear graph that left ample room for possible answers. Just pulling the numbers out of a word problem made it trivially easy.

  45. TurboFool says:

    Yay, I get to feel smart! Got it in about ten seconds.

  46. adora says:

    This is way too easy. Felt a bit insulted.

  47. KentishFerret says:

    OK, I’ve got this one. It took me about 1 minute after spending some time trying to work out figures on a piece of paper before realizing what the answer is.

  48. Rebecca says:

    Got it straight away, very easy.

  49. Hyanur says:

    very easy one… less than a minute

  50. Got it right away, spent a few minutes trying to figure out if I’ve missed something in the question, but I’m pretty sure I’m correct.

  51. Mike says:

    How long to bacteria live for?

  52. irondanimal says:

    This is not as simple at it first appears. Only the bacteria around the edge would be able to grow and divide, so perhaps this problem could have been better presented…

  53. dannyness says:

    less than a minute.

  54. byrde says:

    Less than a minute…assuming I’m right.

  55. Jason says:

    This feels less like a riddle and more like a word problem from my 9th grade math class. Still love the site, keep it up!

  56. Graey says:

    I assume I got it right away. But I’ve thought that before and got the wrong answer.

  57. Dave says:

    Very easy, got it wrong, then right and slapped myself, all within the space of 2 seconds.

  58. Audrey E says:

    It took me about five seconds.

  59. sarah says:

    yea I did what dave did lol x

  60. Gib says:

    I’ve got two answers, both of which took a second. It depends on what happens when the bacterium divides in two.

  61. Rex says:

    Came up with a wrong answer within two seconds, then I looked at the puzzle again and got the right one.

  62. Michael K Gray says:

    I was first introduced to this little puzzle in a talk given by David Suzuki, to indicate where our intuitions about exponential growth fail badly.

  63. spark0ut says:

    Zero time, didn’t even require any thinking about, it is so obvious

  64. Aaron says:

    very confident i have the right answer. took me all of 2 seconds to figure it out, i love logic.

  65. Iis says:

    It shouldn’t take too long, The tricky part will be to find a bacteria that will divide every minute.

  66. Martin says:

    The old waterlily problem.

    Must we assume that no bacteria are ever piled two or more deep?

  67. […] It’s the Friday puzzle! Well, what a week.  I am in NYC now for a few meetings and a skeptics conference. Thanks to everyone who contributed […] […]

  68. Tom Rhoads says:

    I got it immediately. I hope that doesn’t give it away.

  69. Ms G says:

    Hm … why does no one really post the answer ?
    Anyhow .. I would first think .. I’m not gonna work my brain at all … vaguely seeing lots of difficult and energy taking mind calculations .. and then .. sitting in my not too smart self .. I saw it .. at least .. I thought .. but then, if you start thinking … you know that not all bacteria will divide .. so it will take more than a second .. how much .. reality will show . Ofcourse .. statistics would come up with chances stuff and calculations and bla bla bla .. (you know, the derren brown theory on e.g. lottery predictions .. ).

    Now then .. I’m in a box here and I can’t get out … it’s sunday morning .. and I’m bored .. as usual .. so that’s why I am here .. I guess .. And also because Brown Towers could not keep it up …. cheap server management … ehehe …

  70. Martin says:

    Read the top of the page:

    “As ever, please do not post your answer, but feel free to say if you have a solution and how long it took. Answer on Monday.”

  71. Jayden says:

    This is the only puzzle I think I have got right 🙂

  72. Flavio says:

    Took me about 1 second… but in all fairness I solved the same problem (just with leaves covering a lake) years ago. That time it took me some more seconds 🙂

  73. Missy says:

    I’m with everyone else: it took about point zero zero zero … seconds.
    It was so easy I went back to make sure of the details. Was it supposed to be hard??

  74. Niki says:

    Just started following your blog/twitter. Like the sound of these puzzles. I know you’ve already published the answer somewhere but I didn’t look and I did get this one pretty much the moment after I read it. It’s not that hard, but still quite nicely elegant…

  75. maninalift says:

    ummm… I saw the puzzle after this and wanted to see whether the previous puzzle would be as dumb, it seems so. Sorry for being a troll but I dispair at this.

  76. Sakura says:

    This one took me zero time, it’s pretty obvious in my opinion. Just make sure that you’ve read it carefully.

  77. LOl says:

    Wow that was eay just took me 2 secs

    ANSWER- if every cell takes 1min to copy it self, so if it half full then every cell will double and 2 halves make a whole.

    one minute later the dish will be full 47mins+1min = 48mins past Noon = 12:48

  78. fish pond calculator…

    […]It’s the Friday puzzle! « Richard Wiseman[…]…

  79. Cornelius says:

    This stopped working tonight!!! BOOOOOOO!!!!

  80. Anonymous says:

    what is it

  81. Vinod says:

    48th minute it will be full

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