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A Surprize quiz next week


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11 replies to this topic

#1 ujjagrawal

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Posted 04 July 2012 - 11:41 AM

A Logic Class Professor declares on Friday: "We're going to have a surprise quiz next week, but I'm not telling you what day... if you can figure out what day it will be on, I'll cancel the quiz."

The students get together and decide that the quiz can't be on Friday, as if the quiz doesn't happen by Thursday, it'll be obvious the quiz is on Friday. Similarly, the quiz can't be on Thursday, because we know it won't be on Friday, and if the quiz doesn't happen by Wednesday, it'll be obvious it's on Thursday (because it can't be on Friday). Same thing for Wednesday, Tuesday and Monday. So it can't be on ANY day, so there's no quiz next week!"

They tell the professor, who smiles and says, "Well, nice to see you're thinking about it."

On Tuesday, the professor gives the quiz, totally unexpected!

What's the flaw in the students' thinking?
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#2 psykomakia

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Posted 04 July 2012 - 03:03 PM

Spoiler for xodarap

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#3 Aaryan

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Posted 04 July 2012 - 03:30 PM

Spoiler for

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#4 crossquill

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Posted 04 July 2012 - 04:06 PM

A Logic Class Professor declares on Friday: "We're going to have a surprise quiz next week, but I'm not telling you what day... if you can figure out what day it will be on, I'll cancel the quiz."

The students get together and decide that the quiz can't be on Friday, as if the quiz doesn't happen by Thursday, it'll be obvious the quiz is on Friday. Similarly, the quiz can't be on Thursday, because we know it won't be on Friday, and if the quiz doesn't happen by Wednesday, it'll be obvious it's on Thursday (because it can't be on Friday). Same thing for Wednesday, Tuesday and Monday. So it can't be on ANY day, so there's no quiz next week!"

They tell the professor, who smiles and says, "Well, nice to see you're thinking about it."

On Tuesday, the professor gives the quiz, totally unexpected!

What's the flaw in the students' thinking?


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#5 crossquill

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Posted 04 July 2012 - 04:09 PM

As a teacher you never chose Monday...to many students absent or have a week-end hangover:)! Tuesday is perfect because it gives the teacher Wednesday and Thursday to grade the quiz and post it on Friday!
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#6 dpalmer

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Posted 04 July 2012 - 04:54 PM

Spoiler for A solution

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#7 bonanova

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Posted 05 July 2012 - 06:14 AM

This is a variant of the Unexpected Hanging paradox described by Martin Gardner and others.

It was posted in this forum here:
http://brainden.com/...--/#entry128461

And here:
http://brainden.com/...p/topic/1007--/

Please read these threads, and post any new thoughts here.
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The greatest challenge to any thinker is stating the problem in a way that will allow a solution.
- Bertrand Russell

#8 ujjagrawal

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Posted 05 July 2012 - 10:41 AM

This is a variant of the Unexpected Hanging paradox described by Martin Gardner and others.

It was posted in this forum here:
http://brainden.com/...--/#entry128461

And here:
http://brainden.com/...p/topic/1007--/

Please read these threads, and post any new thoughts here.

Fully agreed with Bonanova, this problem is a classical paradox...

As suggested, my thoughts on the same....

The situation in this problem is a contradiction.

Logic very well explain us, there can't be a surprise test next week as then it won't be a surprise.

But, in real world, a surprise is a surprise, if logically deducible that won't be surprise any longer.

The way Professor has declared about Surprise quiz was totally contradictory on logical basis, it is like saying ' you will have a surprise quiz, tomorrow (logically not sensible).

Being heard about the such a surprise quiz, the students should have taken it seriously :) ... should have started preparing their subjects... hehehe...

There is a similar paradox. I will like to put here for all you to give a thought...

Once in a town there was a male barber who, every day, shaves every man who doesn't shave himself, and no one else.

Do you think such a Barber could ever exist ?
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#9 psykomakia

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Posted 05 July 2012 - 01:17 PM

Once in a town there was a male barber who, every day, shaves every man who doesn't shave himself, and no one else.

Do you think such a Barber could ever exist ?


Spoiler for The Barber...

Edited by psykomakia, 05 July 2012 - 01:19 PM.

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#10 psykomakia

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Posted 05 July 2012 - 01:22 PM

Spoiler for Another take on the Quiz...

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