I am speaking about the psychology of dreams at the British Science Festival 2012 in Aberdeen at the weekend.  Do come along and say hi.  Details here.

OK, here is the puzzle.  As ever, please do NOT post your answers, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.

If you turn this number upside down or reflect it left-to-right you get 11 either way, but the original number is not 11.  What is it?

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. The answer is pretty good. Best one in a long time I’d say even though I got it in a few seconds, it’s the type of thing that you could easily miss.

  1. I’ve got a good answer which is nothing to do with binary, and I don’t get the comments ’11’ in binary is 3, so what’s that got to do with anything?

  2. I have two answers. The first one, which I think is the one you will reveal on Monday only took a few seconds to get. The second answer I got after reading some of the comments – it is also correct given the way the question is phrased.

  3. It took me about 5 minutes, but I am not sure if it is right or not. It depends a little on what “turn it upside down” means. My answer doesn’t work if it means a mirror reflection, but if it means writing the answer on a piece of paper and turning it through 180 degrees it works

  4. It took 4.3 seconds to come up w/an answer. It took a full minutesnd a half trouble convince myself the answer was too easy and that I have an 83.8% chance of being wrong.

  5. The problem says “If you turn this number upside down or reflect it left-to-right you get 11 either way, but the original number is not 11. What is it?” Does Richard mean that the number in question is represented as “11” or that “11” is the number “eleven”?

    1. My interpretation is that Richard’s first “11” means the integer between ten and twelve. The second “11” means the symbol “11”.
      There is another solution as well if you interpret the question differently. Again you have to ascribe two different meanings to the symbol “11” in the question to make it work.
      In other words it is a trick question.

    2. I see what you mean now Match.
      i now how three answers, with my third being the one I expect RW to post.
      The comment below about the clear wording of the question helped.
      My other two answers and my comments are still acceptable within the normal parameters of WisemanWorld.

  6. 9 secs to get an answer then 9 mins trying to make sense of the comments,
    I can also do it ‘by moving just one of the matchsticks’

  7. I think the wording ‘…turn this number upside down…’ is ambiguous as it does not say about which axis the rotation should be made. Rotation about an axis normal to the plane of the screen gives a different result to rotation about a horizontal or vertical line on the screen.

    1. II = 2 in Latin. Any Roman knows that whether it’s upside down, or right side up, or rotated left or right. SPQR

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