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.

100 pennies are on a table.  10 are tails.  With your eyes closed, how can you separate the pennies into 2 groups such that each group has the same number of tails?
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.

57 comments on “It’s the Friday Puzzle!

  1. Geoff the God of Biscuits says:

    Sorted in 2 seconds, unless I missed something. #irock #best #waitthisisnttwitter

  2. SofARMaths says:

    A few seconds – or am I telling tails…

  3. thejamesbb says:

    can i count it in nanoseconds? #woopwoop
    classic puzzle.

  4. The Masked Twit says:

    About a minute.
    I think Richard has posted this one, or something similar before.
    Good puzzle.

    • Julia says:

      last time it was cards. I remember thinking it was clever then, but I can’t remember how it was done.

  5. Brian A says:

    Oh, very good… less than a minute – once I’d eliminated a couple of daft guesses.

  6. André Prata says:

    You posted essentially the same question a while ago… January, iirc…

  7. Anonymous says:

    3 secs and I realised I was never going to work this out. I’ve been at it for an hour and I still haven’t got a clue.

  8. the great me says:

    20 minutes,
    nice one

  9. Selim says:

    A few seconds 😉

  10. Paul Durrant says:

    I vaguely remembered this puzzle, but it took a minute to work out the solution again.

  11. Stevie. says:

    Just over a minute, as I wanted to check my thinking was correct.

  12. Dominic says:

    Brilliant puzzle!

  13. Baker's Dozen says:

    By asking a friend to do it?

    Another stupid puzzle I’m afraid.

  14. C Thorn says:

    Easy – a few seconds to think of the way to do this and a minute to test it.

  15. Anonymous says:

    I got ‘an’ answer in a couple of seconds, but it might not be ‘the’ answer.

    • Anonymous says:

      I’ve also thought of a second different answer in a few more minutes, that almost certainly isn’t ‘the’ answer but has a pretty decent chance of success.

    • Tom says:

      I think you need to wait for the answer before deciding whether it is stupid or not.

  16. Heather wiseman says:

    Count two piles of 50!

  17. ctj says:

    a few seconds for one solution that requires the use of US pennies.
    about the same for an ontological “solution.”
    a little more for the intended solution.

    does anyone have more than three solutions?

    • Lazy T says:

      I trained my pipistrel bat to differentiate them by echo sounding, then to tell me one squeak for heads, two squeaks for tails.
      What’s so special about US pennies?

  18. OldManLink says:

    10 minutes. Lovely puzzle, thanks!

  19. Anonymous says:

    10 seconds…like it

  20. Anonymous says:

    Almost immediately (if I’m right!)

  21. Anonymous says:

    I’ve got three different answers to this, all of them legitimate.

  22. As a member of the now now now generation I had a few second thought and gave it up as impossible. I look forward to Monday…

    • Dave says:

      Huh, a self-professed “member of the now now now generation” who didn’t even think to google it?

  23. Liam says:

    I’m totally missing what the challenge is here? Are you not supposed to be able to feel the difference between the coins?

    • Dave says:

      I would have thought that was blatantly obvious.

    • One Eyed Jack says:

      It’s not forbidden by the puzzle description, so it is a valid solution.

    • Anonymous says:

      Well that wouldn’t make much of a puzzle. Just pretend Richard said you can’t tell the difference by touching.

    • Vaahto says:

      If some people had their way, you’d need a 50 page description accompanying each puzzle to clearly define the circumstances.

  24. The Pick Man says:

    How long does it take to say, ‘Zip’?

  25. Okay, the ‘correct’ answer took a couple of seconds, BUT . . . (and I know we should not post answers, but this is clearly not the puzzle answer) a very short bit of testing shows that i can reliably recognise which way up a coin is, using only my fingers.

    I could very easily brute force this.

  26. Heidi says:

    Literally no time at all.

  27. Jim says:

    Classic puzzle is right, and all math guys like me know it. I still don’t understand why it works. It took me less than 0.1/10^(google plex) nanoseconds to remember the solution.

  28. Steve Augustin says:

    I think that there is insufficient information to solve this properly i.e. the problem is not ‘locked down’ enough. Maybe that’s Richards idea. In a vacuum all sorts ‘ideas’ and assumptions will pop up. I haven’t a clue how to solve it other than to feel the pennies to see which is heads and which tails. No one said that this was not allowed but me thinks that that is one of the unstated not allowed solutions. I don’t like it the a problem is insufficiently stated. It leads to messy and loose thinking.

    • Jim says:

      Believe it or not, Steve, there really IS a mathematically correct and logical answer to the puzzle. If you send a note to me ( I’ll reply to you with the solution. This is one of my favorite math problems. –Jim

    • Abu's Hamster says:

      Just think of it as a thought experiment.

  29. John says:

    Easy peasy lemon squeey

  30. Kristian says:

    I think we’ve seen this before (maybe years ago, I admit), but I’m actually okay with recycling the good ones 🙂

    Great puzzle I think, because it can seem impossible at first, but the solution is very elegant. Took me a long time to solve the first time.

  31. Phil H says:

    After getting over the frustration of not remembering the similar puzzle before, it didn’t take long. So, that’s about 4 hours 🙂

  32. Niva says:

    0 secs since I heard this puzzle (and similar ones) before. This particular one was just on Cartalk! I guess Richard must listen to the podcast.

  33. john jahanpour-burke says:

    I Did This In 2 Seconds Because I Heard This One Before On NPR.

  34. Good one! It took me about a half a minute to see a possible solution and then another half minute to convince myself that it really works.

  35. As ever, I have thought about the answer before reading through the thread (so I don’t yet know whether there are any spoilers in the thread). Almost immediately I had an idea of what the “trick” might be to make it work, but i had to think for about 3 minutes before I had worked out the exact details of what to do.

  36. Dan Broderick says:

    All you people boasting about solving this in a second or 2, you are all merely remembering the puzzle from when you heard it before, you’re not solving it that quickly- get over yourselves

  37. rmb says:

    June 18, 2012

  38. Dharmaruci says:

    surely this is no puzzle at all. just cut each penny in half.

  39. Henry says:

    Did it with mittens on!

  40. Rachel says:

    First time visit here and have your great article. May I’ve a copy of this post?

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