Motus Mentis

7 Comments

1. Kevin Kim says

   It doesn’t help that the logic puzzle is written in Chinglish. I’ll get back to you soon (I hope) with an answer. Assuming I can puzzle through the awkward English.

   Posted April 14, 2015 at 12:07 pm | Permalink
2. Malcolm says

   It’s all right there. (Yes, it should be “became friends”, and Bernard should say ‘at first I didn’t know”, but other than that, the rest is perfectly fine.)

   Posted April 14, 2015 at 12:15 pm | Permalink
3. Kevin Kim says

   A more proper rendering:

   Albert and Bernard have just become friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of ten possible dates.

   [dates]

   Cheryl then tells Albert and Bernard separately the month and the date of her birthday, respectively.

   Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard doesn’t know, either.

   Bernard: At first, I didn’t know when Cheryl’s birthday was, but now I know.

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

   So when is Cheryl’s birthday?

   _______________________

   Just had to put that out there.

   Posted April 14, 2015 at 12:31 pm | Permalink
4. Kevin Kim says

   Never do these at 3 in the morning. I watched Mr. BBC’s explanation, which made perfect sense. The Facebook commentary, by contrast, was all over the place, but I noticed that some Indian commenters used raw arithmetic to brute-force their way to the BBC guy’s solution. That was pretty impressive, even though I don’t quite understand the Indian commenters’ starting assumptions.

   Posted April 14, 2015 at 12:53 pm | Permalink
5. Dom says

   Reminds me of this puzzle.

   Malcolm and I have numbers on our foreheads. I can see his but not mine, he can see mine but not his. We only know that the numbers are between 1 and 100 and they are consecutive. After looking at each other’s numbers we respond with “I know my number” or “I don’t know my number”.

   1. I am looking at 100; Malcolm is looking at 99. I say, “I know my number”, Malcolm thinks “My number is either 100 or 98. If Dom knows, then mine must be 100. H say, “I know my number too.”

   2. I am looking at 99; Malcolm is looking at 100. I say, “I don’t know”. Malcolm says, “I know (see case 1)”. I say, “Then I do know”.

   3. I am looking at 99; Malcolm is looking at 98. I say, “I don’t know’. Malcolm says, “I don’t know”. I say, “Then I know”. Malcolm says, “Then I know too.”

   This can play out indefinitely. That is, if I am looking at, say, 14, and Malcolm is looking at either 13 or 15, there will be a string of “I don’t know”, “I don’t know” … until one says, “then I do know” and the other says, “Then I know too”. It is logically impossible for both of us to not know the number on our foreheads.

   Posted April 14, 2015 at 6:32 pm | Permalink
6. TheBigHenry says

   Reminds me of this joke:

   Guy goes into a store, walks up to the counter and orders a kielbasa (a Polish sausage). The store owner says, “You must be Polish.” The guy goes ballistic: “If I ordered an Italian sausage you would assume I was Italian?!” The store owner replies, “Not really.”

   Question: What kind of store is it?

   Answer: It’s a hardware store.

   Disclaimer: I was born in Poland, so I have permission to tell this sort of joke. Please don’t call the PC police.

   Posted April 15, 2015 at 12:42 pm | Permalink
7. Malcolm says

   Dom, that’s like the “surprise quiz” story:

   The teacher tells the class there will be a quiz sometime next week, but he won’t say what day it will be. It will be a surprise.

   But of course it can’t be Friday, because if Thursday goes by with no quiz, then everyone knows it will have to be Friday. So you can’t have a surprise quiz on Friday.

   But if Friday’s out, then the last possible day for a truly “surprise” quiz is Thursday.

   But this means you can’t have a surprise quiz on Thursday either, because if you get through Wednesday with no quiz, you know it has to be coming on Thursday (we already know Friday is off the table). So Thursday’s out, too.

   …and so on.

   As it happened, the teacher did give the quiz, on a randomly selected day of his choosing, and surprised everybody.

   Posted April 15, 2015 at 1:36 pm | Permalink