When Is Cheryl's Birthday? • Hacker News Board

Re: When Is Cheryl's Birthday?

#101

Post by andrewprock » Sat, Apr 25, 2015, 3:06 PM UTC

stephen_g wrote: post #65

andrewprock wrote: post #61

Earlier quoted context omitted.

IIRC, if Bernard speaks first, then the correct answer is August 17.

That doesn't make any sense to me. Can you post exactly the sequence of statements if Bernard speaks first? Simply reordering the questions does not make sense, so I can't see it happening any way other than what I posted, which shows that it wouldn't change anything.

It really depends on who says what, but here a version of the problem where Bernard speaks first:

Bernard: I don't know when Cheryl's birthday is.

Albert: I don't know when Cheryl's birthday is.

Bernard: I know when Cheryl's birthday is now.

Albert: Then I also know when Cheryl's birthday is.

Re: When Is Cheryl's Birthday?

#102

Post by thuffy » Sat, Apr 25, 2015, 3:19 PM UTC

anigbrowl wrote: post #32

andrewprock wrote: post #13

Unfortunately, there are a number of rather curious, and culturally specific, assumptions one must make about the problem. The real puzzles are: 1. "Why did Albert speak first?" 2. "Why did he speak in such cryptic language?"

These are actually valid questions and you shouldn't be downvoted. The standard interpretation of the problem is highly questionable; Albert's initial remark is not actually dispositive to Bernhard, because if Bernhard had the 19 May date he would know the correct answer immediately without needing Albert to say anything. The suggestion that Albert's first statement necessarily eliminates all dates in May is false, a…

Solved this without even a pencil and paper, just in my head, in under 60 seconds while being distracted by another person in the room asking me what to have for breakfast. There is only one possible answer as the others pointed out; you are definitely wrong that the answer is indeterminable.

The whole ruckus surrounding this problem seems to be more a reflection of the sad state of the intelligence in our population, rather than anything to do with Asia.

Re: When Is Cheryl's Birthday?

#103

Post by cynicalkane » Sat, Apr 25, 2015, 3:28 PM UTC

codeulike wrote: post #44

baddox wrote: post #40

Earlier quoted context omitted.

Which part is the trick? It all seems very straightforward to me.

Its understanding what the question actually means when it says things like 'Bernard: At first I don't know when Cheryl's birthday is, but I know now.' ... this is actually a cryptic coding of the rules of the puzzle. If you properly understand those rules and convert them to code (as in the OP) then searching the possible answers for a match is trivial.

It's Singapore English. Instead of using tenses, you put a phrase expressing time at the beginning of a clause, a feature borrowed from Chinese. "At first" sets the time and topic of the following clause. Having spent some time in Singapore and HK, I found the Chinese-flavored English to be immediately clear. Actually, if you just guess to substitute the correct verb tense (don't -> didn't), it seems clear to me in ordinary English as well, and I'm not sure where the confusion lies.

The Singaporean government doesn't think Singapore English is a valid thing and wants everyone to learn "good" English. The Singaporean upper class is, as usual, on the government's side, and will complain about the poor English education in their country. That's fine for them, but please be aware that when you say "this is bad English" you are making a cultural and class-based judgment. What you really mean is the math question uses a way of expressing time that is familiar to them but not familiar to you.

Re: When Is Cheryl's Birthday?

#104

Post by TillE » Sat, Apr 25, 2015, 3:33 PM UTC

Nursie wrote: post #99

s_kilk wrote: post #95

Earlier quoted context omitted.

This is exactly why I find these kinds of puzzles infuriating. It always hinges on some contrived, bullshit "trick" which has nothing to do with solving real problems.

It's really not a trick. You simply take all the knowledge you are given and (this is important) don't make any additional assumptions or guesses or even really think of the participants as human, and.solve it like a math problem.

Yeah. I'm completely unable to do any of the stereotypical interview question puzzles which hinge on an "outside the box" trick, but I had no trouble recognizing this as a straightforward problem that you just work through.

Re: When Is Cheryl's Birthday?

#105

Post by jmilloy » Sat, Apr 25, 2015, 3:49 PM UTC

baddox wrote: post #46

codeulike wrote: post #44

Earlier quoted context omitted.

Its understanding what the question actually means when it says things like 'Bernard: At first I don't know when Cheryl's birthday is, but I know now.' ... this is actually a cryptic coding of the rules of the puzzle. If you properly understand those rules and convert them to code (as in the OP) then searching the possible answers for a match is trivial.

Why do you say that is cryptic? I can't think of a clearer or plainer way to express that fact.

I think if you were actually tasked with finding a plainer way to express the fact, you would have no trouble.

When Albert says I don't know when Cheryl's birthday is, but I know that Bernard does not know too, consider There are no unique days in the month I was told.

Informally, note the difference of order. If Albert said "I was told the Birthday is in July", that would be, say, first order. The plain version I provide is a logical statement about the months and dates, a second order statement. But the cryptic version is a statement about what (second-order) statements Bernard can make, so third order. Hence, cryptic.

Re: When Is Cheryl's Birthday?

#106

Post by stephen_g » Sat, Apr 25, 2015, 4:01 PM UTC

andrewprock wrote: post #101

stephen_g wrote: post #65

Earlier quoted context omitted.

That doesn't make any sense to me. Can you post exactly the sequence of statements if Bernard speaks first? Simply reordering the questions does not make sense, so I can't see it happening any way other than what I posted, which shows that it wouldn't change anything.

It really depends on who says what, but here a version of the problem where Bernard speaks first: Bernard: I don't know when Cheryl's birthday is. Albert: I don't know when Cheryl's birthday is. Bernard: I know when Cheryl's birthday is now. Albert: Then I also know when Cheryl's birthday is.

That breaks the whole problem because you've removed the part where Albert says something that gives Bernard the information he needs to work out the date.

I don't think it's really a good critique of a logic problem to say, "but if you remove this extremely important part, it doesn't work anymore"!

The conversation would need to be:

Bernard: I don't know when Cheryl's birthday is

Albert: I don't know when Cheryl's birthday is, and I knew that Bernard didn't know.

Bernard: Then I know when Cheryl's birthday is now.

Albert: Then I also know when Cheryl's birthday is.

Then it works. You only have to assume that the parties are logical and tell the truth.

Re: When Is Cheryl's Birthday?

#107

Post by xarien » Sat, Apr 25, 2015, 4:17 PM UTC

I'd have gone a different way since my setup and approach was a bit different (I took a more visual approach). While this solution is elegant, it's also incredibly hard to do in the real world as it requires far too much knowledge to properly assign the "correct" definitions. It's no different than having a perfect health heuristic land on your lap.

When I did this a while back, my approach was to construct a 5x4 matrix with blanks (5 unique dates x 4 unique months). This allowed the clues to cross off entire rows and or columns until only 1 pair stood standing. Personally, I find the construction of the solution much more interesting than the problem itself given how many people participated.

Re: When Is Cheryl's Birthday?

#108

Post by Nursie » Sat, Apr 25, 2015, 4:19 PM UTC

anigbrowl wrote: post #32

andrewprock wrote: post #13

Unfortunately, there are a number of rather curious, and culturally specific, assumptions one must make about the problem. The real puzzles are: 1. "Why did Albert speak first?" 2. "Why did he speak in such cryptic language?"

These are actually valid questions and you shouldn't be downvoted. The standard interpretation of the problem is highly questionable; Albert's initial remark is not actually dispositive to Bernhard, because if Bernhard had the 19 May date he would know the correct answer immediately without needing Albert to say anything. The suggestion that Albert's first statement necessarily eliminates all dates in May is false, a…

If we take silence as indicative of lack of knowledge, then nobody needs to speak.

  Bernard: Silence (he doesn't know the answer)
  Albert: Silence (he doesn't either)
  Bernard: I know it!

Interestingly it then becomes relevant who was silent first.

Basically it's nonsense.

Re: When Is Cheryl's Birthday?

#109

Post by andrewprock » Sat, Apr 25, 2015, 4:56 PM UTC

stephen_g wrote: post #106

andrewprock wrote: post #101

Earlier quoted context omitted.

It really depends on who says what, but here a version of the problem where Bernard speaks first: Bernard: I don't know when Cheryl's birthday is. Albert: I don't know when Cheryl's birthday is. Bernard: I know when Cheryl's birthday is now. Albert: Then I also know when Cheryl's birthday is.

That breaks the whole problem because you've removed the part where Albert says something that gives Bernard the information he needs to work out the date. I don't think it's really a good critique of a logic problem to say, "but if you remove this extremely important part, it doesn't work anymore"! The conversation would need to be: Bernard: I don't know when Cheryl's birthday is Albert: I don't know when Cheryl's b…

You have to make quite a few more assumptions as well. If they were logical and told the truth, the conversation would be more like:

Bernard: Cheryl told me it's the 17th.

Albert: Cheryl told me it's in August.

Re: When Is Cheryl's Birthday?

#110

Post by baddox » Sat, Apr 25, 2015, 6:20 PM UTC

chongli wrote: post #83

baddox wrote: post #69

Earlier quoted context omitted.

Why do you think that confusion over the English description is more to blame than the difficulties many people face with logic puzzles?

A lot of people reading it probably didn't even realize that it is a logic puzzle. To those, it may be some kind of obscure riddle with terrible grammar. Heck, it's not even clear that Albert and Bernard are communicating with one another and not merely with the answerer. If you assume they do not communicate at all then the puzzle becomes impossible.

> A lot of people reading it probably didn't even realize that it is a logic puzzle. To those, it may be some kind of obscure riddle with terrible grammar.

I'm not clear on what the difference between a riddle and a logic puzzle would be in this context.

> If you assume they do not communicate at all then the puzzle becomes impossible.

Why would someone make an assumption that renders the puzzle impossible? I suppose the questioner could just be a troll or something.