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

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

  1. 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.

    1. 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.

  2. 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?

    1. 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?

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

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

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

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

  4. 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.

  5. 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.

  6. 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.

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

  7. 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.

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

  9. 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.

  10. 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.

  11. 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

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