# It’s the Friday Puzzle!

68

@jbrownridge sent me this lovely puzzle yesterday….

If you choose an answer to this question at random, what is the chance that you will be correct?

a) 25%
b) 50%
c) 60%
d) 25%

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.

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.

## 68 comments on “It’s the Friday Puzzle!”

1. Bob O'H says:

Ha! I like it.

2. Mickey D says:

Have to think a bit more…

3. Rusty says:

This may be my favourite Friday Puzzle so far. I have that #MyBrainIsBeingMessedWith tingle. Also – how can it possibly be Friday again already? I could have sworn it was Tuesday!

4. Mad Kev says:

It’s a trick. There’s no such thing as random.

• Steve Jones says:

I see you are not a believer in quantum mechanics

• diavoli says:

Heisenberg would have been p*ssed off to find out.

5. Goliath says:

1min… with the ever so slight feeling I might be completely off track. As usually with these kind of puzzles.

• Goliath says:

To add: is the random answer to this multiple choice question evaluated by a computer or a human being?

6. Kristian says:

If you want to avoid all the weirdness about probability I think this is a sensible rephrasing of the puzzle:

How many option(s) contain the correct answer?
a) 1
b) 2
c) 3
d) 1

Quite funny puzzle, it also made it to the frontpage of reddit for a short period of time ( http://www.reddit.com/r/funny/comments/lr1rw/probability_mindfuck/ don’t worry, comments don’t seem to contain any sort of actual answer).
I think I’ve got an answer, took me a minute or two to think it through.

• Invisiblesilence says:

That is completely different question, as it does not contain a meta element.

7. JimC says:

I think I got this quite quickly, but I’m open to the possibility that I might be completely wrong.

• The Pick Man says:

Good for you, Jim.

“Humility is the solid foundation of all virtues.” – Confucius

8. Steve Jones says:

I assume in the absence of other information we have to assume that the three values of 25%, 50% & 60% are the only possible ones and all have equal probability of being right. Also, in making a random choice, it’s not a random choice of the three values but of the four options (as two answers are identical). I know this sounds pedantic, but problems of chance like this are predicated on precise definitions.

Given that, I have an answer (which, unusually, matches my first instinct).

• Steve Jones says:

nb. without some assumptions this becomes an exercise in the meaning of what is meant by correct or a question.

9. Michael Sternberg says:

A couple of minutes to break out of the self-referrals.

Speaking of which, surely the answer depends on the exact referent of “correct”.

10. edgar1975 says:

Well, what’s the question? It says “If you choose an answer to this question at random”, and it’s followed by “what is the chance that you will be correct?” But what is the question in the first place?

• That is the question: “what is the chance of randomly guessing the correct answer?” That is it, which is what makes this such a clever and funny puzzle. The self referral leads to all sorts of paradoxes, but I’m not sure there is actually an answer. I hope I’m wrong though.

• Oh wait, now I have an answer that works. Although in this case the puzzle is much more beautiful and satisfying than the answer because it’s just so clever.

11. Anonymous says:

Since in the text of the question we do not have a decision criterion to evaluate the correct answer we have to assume that every answer is correct. So the chance to be correct is 100%.

12. Steve Jones says:

An interesting idea for this blog would be a way for people to provide their answers in a way that won’t be revealed until the following Monday. That way we might find out how many of those who claim to have got the right answer actually have. Or is that me being slightly cynical…

13. lehcyfer says:

Correction, 5minutes
Sneaky 😛

14. lehcyfer says:

Steve – that’s what Richard would love – he’s a scientist after all…
Some kind of voting system would be perfect – there are voting platforms on the web that provide just that.

Richard, follow up on that – you’re wasting a lot of useful data here…

15. Jamie says:

Got ‘my answer’ in around 2-3 minutes upon second reading. Not to say it is the right answer but I have one.

16. sohvan says:

At first I thought it was an impossible question for quite some time. Then I saw a possible answer. Very clever.

• Carl says:

I’m intrigued by this. I would have given up ages ago and declared it impossible but for comments like this. I hope it’s worth persevering with.

17. I guessed the wrong answer immediately, but then I read the question more carefully and had a light bulb moment a couple of minutes later. Might still be wrong, of course.

18. Lore says:

If I’m correct I got the answer by the end of the question,
but usually I have to think a lot about the puzzles, so maybe I’m wrong.

19. Jamie says:

Is it me overthinking this further or could the answer keep bouncing back and forth because of the double question/answer scenario?

• Carl says:

That’s what I think too. But I’m going to mull it over some more.

20. Anonymous says:

Regardless of my choice of solutions provided, I cannot be wrong due to the fact that I am always right. ha! XD

21. Scott Halls says:

My brain!!

22. A Duck.

23. Tom Pedersen says:

My brain just broke, I’ll send you the invoice for a new one.

Also, this sentence is false.

24. mittfh says:

A few seconds to come up with one answer, another few seconds to re-evaluate it and choose a different option which I believe is correct. This has to be the easiest one I’ve answered to date…

…unless Richard’s double bluffing us!

25. Lazy T says:

Good question, loads of answers, but theres no chance of me getting it correct, coz I’m always wrong.

26. Camel Ali says:

I like this puzzle. Had to give it a bit of thought this time, hence the contorted look on my face and the loud clanking noise!

27. Shecky R says:

I too like this puzzle. Moreover, I like that I like it. In fact, I like that I like that I like it. I even…….

28. Ann Kittenplan says:

All Cretans are liars

• Michael Sternberg says:

… and the shave only those who do not shave themeselves.

29. David Hanley says:

A couple seconds to come up with the obvious answer, a few minutes wondering if there’s a trick.

30. Took me some time to figure it out, but I have an answer.

31. sailor says:

Easiest one yet. Its not the first obvious solution that occurs to you but the second.

32. Gus Snarp says:

You just blew my mind. It’s like infinite recursion, I can’t process it….

33. markstanbrook says:

Oh! When I unravelled the self-referential nature of the question and answers it became obvious. Very clever. 2 minutes.

34. Drew says:

Very good! I like this one

35. Logic, meet circle. Circle, logic.

36. John Loony says:

This one (unlike most Friday puzzles) is obvious. It only took about 5 seconds.

37. Dannie says:

If anyone follows the Post Secret Blog, they just posted this on their facebook page! Quite a few people came up with the same answer as me.

38. alvin says:

took me 10 seconds

• Steve Jones says:

I think maybe visiting the doctor is in order.That doesn’t sound either natural or very satisfactory.

39. fluffy says:

Heh, this was on the front page of Stack Exchange yesterday.

40. dharmaruci says:

note how clevver mr wiseman is.

the question mark is before the four opttions.means the options are a complete redherring. nothing to do with the question at all.

the fulll query is “If you choose an answer to this question at random, what is the chance that you will be correct?”

the answer to that is [spoiler]

• Carl says:

Good point, but the same problem still exists. I.e. If you’re not restricted to one of the four possibilities, then you can basically go with any probability, which provides infinite options. Which means the chance of you getting it right is 0. But then if you choose 0, then you’ve got it right, falsifying your choice. So the question is still unanswerable in the same, self-conflicting way.

41. FrankN.Stein says:

It’s..
No wait it’s…
Now it’s….
Arrrrrrrrgggggghhhhhhhhh

42. TS says:

Software failure. Press left mouse button to continue.
Guru Meditation #00000004.00C13870

43. Will says:

Why does this problem bring to mind lessons on GAAP?

44. John Donachie says:

The 56 comments do no justice to the fun being had over on google plus with this question http://flowingdata.com/2011/10/28/best-statistics-question-ever/

45. Carl says:

Man, I can’t wait to see the arguments on this one come Monday.

46. Mike Farmer says:

This is probably the strangest coincidence I’ve ever experienced. I just saw a photograph of this exact question written on a chalkboard and it reminded me to check this blog for the weekly puzzle. Crazy weird!

47. The reply should be “mu”.

Any fans of Douglas Hofstadter here?

http://dictionary.reference.com/browse/mu

48. pl says:

What if the question has any meaning beyond a boolean answer? Like How are you? Instead of Do you feel fine or bad?

49. pl says:

What if I decide not to choose an answer at random, since I know the correct answer?

50. There are an infinite of random answers that you could give. I could answer ‘sausages’, for example. Therefore, the answer is 1 in infinity.

51. MAG says:

HAY DOS soluciones, veamos :

A) 0% Es la respuesta a la pregunta “Cual es la probabilidad de acertar? “.
0% No está entre las opciones así que nunca acertaria. NO pedian que marquemos una opción y ver si hemos acertado nos han pedido la probabilidad de acertar si escogieramos.

Esta es también la respuesta lógica ante los resultados 25% y 50% que se invalidan en bucle.

Esta opción se fortalece si partimos de que la respuesta puede no estar entre las opciones.

B) La respuesta podría ser 25% o 1 de 4.
Me explico y sigo respondiendo a Cuál es la probabilidad de acertar?

Las posibilidades de que al azar marque un 25% son de 2/4 (a) y (d) o sea una probabilidad del 50% que es la opción b).

Así que la probabilidad de que escoja b) es de 1 de 4 o sea del 25%.

Así que 25% es la probabilidad de marcar b) y decir que tengo 50% de probabilidad de escoger la respuesta correcta 25%.

Esta solución se refuerza si partimos de que la respuesta SI está entre las opciones:)

52. Anonymous says:

La probabilidad de acertar es igual a 1 – la probabilidad de no acertar.
Puesto que cualquiera de las opciones elegidas no es correcta, la probabilidad de no acertar es 1.
Entonces la probabilidad de acertar es 1 – 1 = 0

53. Daniel says:

The chance of hitting is 1 – the probability of not hitting.
Since any of the choices is not correct, the probability of not hit is 1
Then the probability of hitting is 1-1 = 0

54. Marian says:

Because the admin of this web site is working, no question
very soon it will be famous, due to its feature contents.