Logic riddle gone viral (math-ish)

[TheMatrix31]

Re: Logic riddle gone viral (math-ish)

The way this entire thing is written makes me want to slice my wrists before ever attempting to solve it. I've read all these explanations and it still makes zero sense to me whatsoever.

Maybe I'm just not that bright.

[Blzer]

Re: Logic riddle gone viral (math-ish)

The way this entire thing is written makes me want to slice my wrists before ever attempting to solve it. I've read all these explanations and it still makes zero sense to me whatsoever.

Maybe I'm just not that bright.

I said I wouldn't rewrite the problem, but for your sake I'll make a few modifications (for the sake of the wording, "birthday" will mean month & day, while "birth day" will mean number of the day of the month):

Albert and Bernard befriend a girl named Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates that it could be:

May 15, May 16, May 19
June 17, June 18
July 14, July 16
August 14, August 15, August 17

Cheryl then whispers her birth month to Albert and her birth day to Bernard, and ask them to see if they can find out when her birthday is.

The following dialogue ensues:

Albert: "I don't know when Cheryl's birthday is, but based on what I know about her birth month and the list given, I know for a fact that Bernard can't possibly know when her actual birthday is either."

Bernard: "When Cheryl first told me what her birth day was, I didn't know when her actual birthday was. However, after Albert said that he doesn't know and he knows I couldn't possibly know either, I've since narrowed it down to only one date that it could possibly be."

Albert: "Okay Bernard, if that's the case then that means I can whittle it down the same way. I too now know when Cheryl's birthday is."

So when is Cheryl's birthday?


Here's the thought process behind it, more simplified:

Spoiler

Albert is only given a month, so of course he has no clue when Chery's actual birthday is. However, he was given a month of July or August. This is the only way he'll know for certain that Bernard doesn't know the date, because he knows that only May and June have specific dates (the 19th and 18th). If Albert was given May or June, that means he can't guarantee Bernard doesn't know what Cheryl's birthday is. Every other day number given is shared among multiple months, and July/August shares numbers as well.

Bernard understands this concept that Albert must have July or August. If Bernard now knows the date, it's based on the fact that it must the a 15, 16, or 17 (14 appears in both months).

For Albert to now know when it is, he must have July, because in August the 15th and 17th still remain, while July has only one date left. That date is July 16th.

Maybe I'm bad at explaining this, but it's easier if you cross out dates as you go.

[TheMatrix31]

Re: Logic riddle gone viral (math-ish)

That makes way more sense now that you rewrote it.

[LowerWolf]

Re: Logic riddle gone viral (math-ish)

I understand the answer, but here's my issue:

Spoiler

19 and 18 are the only uniques date. So even if Albert didn't know the month, you could rule out May 19 and June 18 based on his statement that he knows Bernard doesn't know.

To me it's a stretch to say you can rule out May completely because May has a unique date.

Maybe I've overthought it though.

[Blzer]

Re: Logic riddle gone viral (math-ish)

I understand the answer, but here's my issue:

Spoiler

19 and 18 are the only uniques date. So even if Albert didn't know the month, you could rule out May 19 and June 18 based on his statement that he knows Bernard doesn't know.

To me it's a stretch to say you can rule out May completely because May has a unique date.

Maybe I've overthought it though.

Spoiler

The idea is that Albert already knows what month Cheryl told him, so all we're trying to do is feed into the clues that he says. What he tells us is: "I know for a fact that Bernard has no chance in 100% knowing when her birth date is." That clue in itself has him saying that he has July or August. Remember, Albert knows the list of days, so he is literally cluing into us what months we can eliminate.

Let's put it this way: if Albert had June, he couldn't make the statement that he knew without a doubt that Bernard didn't know Cheryl's birthday. Cheryl very well could have told Bernard the 18th in that case. We don't know anything about what Bernard knows when Albert makes that statement yet, because Bernard hasn't said anything.

[HealyMonster]

Re: Logic riddle gone viral (math-ish)

I understand the answer, but here's my issue:

Spoiler

19 and 18 are the only uniques date. So even if Albert didn't know the month, you could rule out May 19 and June 18 based on his statement that he knows Bernard doesn't know.

To me it's a stretch to say you can rule out May completely because May has a unique date.

Maybe I've overthought it though.

Spoiler

Thats essentially where I ended. I was left with May 16 and July 16 After I eliminated everything else, I didn't do the whole eliminate May and June 1st as it went over my head, but I knew based off that cause it wasn't the 18th or 19th

[Ghost Of The Year]

Re: Logic riddle gone viral (math-ish)

Normally I love stuff like this, but this one gave me ADHD.