Challenge - Puzzle carries prizes! 3 votes of 100 percent VP.

in #challenge7 years ago (edited)

The challenge is over, the winner is @kouba01 You can still read this wonderful puzzle.


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Josef and Roi became friends of Sarit and they want to know when her birthday. Sarit presents them with a list of twelve possible dates:

January 14, January 15, January 18, February 16, February 17, February 13, February 15

March 13, March 15,

December 13, December 14 and December 16.

Sarit reveals to Josef and Roi, respectively, to each the month and to the other the day she was born.

(We do not know which of them she told what day\month.)

Josef: I do not know when Sarit's birthday is, but I know Ro'i does not know either.

Roi: At first I did not know when Sarit's birthday was, but now I know.

Josef: So I know when her birthday is as well.

So - when was Sarit's birthday?


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Lovely romantic puZZLE

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Holly shit man, what is the answer. I just got back from University after 9 hours of study and not in a condition right now to use my brain again😆

nice post n puzzle i love it 😊

Well, I am not good in Math...

@amiramnoam - Total set of possible combinations of month and dates are :
possible data set:
{J: 14, 15, 18;
F: 13, 15, 16, 17;
M: 13, 15;
D: 13, 14, 16; ) (Thanks to @laurakodi for this summary. I had writting the same summary on a piece of paper but when I started typing my answer, I saw this summary. I thought - perhaps the answer is already solved by someone so I just took the summary and have worked out answer on my own. No issues about whether i get prize or not - It was fun!! Thanks for the brain teaser ) Anyway - here is my logic
Let us make an assumption that Joseph knows month and Roi knows date. For Joseph to say for sure that Roi does not know birthdate for sure, Joseph must know that the month he knows does not have ANY unique date (meaning all the dates in the month he knows have to be present at least in one more month). So, the months Jan and Feb which have 17th and 18th as unique dates are eliminated. Now, Roi, hearing Josephs statement, uses same logic and eliminates Jan and Feb. Then he says NOW he knows for sure. That means the date he has can not be duplicated in both March and December otherwise he would not know the month for sure. So, the dates 13 is eliminated from both December and March. Now, it can only be March 15, December 14 or December 16.
Joseph uses this logic based on what Roi says and then mentions he knows date too so, he is seeing only one date left in the Month he knows. Since this is only in March, the date is March 15.

Hope that logic is correct.

Regards,

@vm2904

The answer is March 15

Roi is given the day and Josef is given the month. This is because Josef can only know the month is he is given a unique month but there are none. Roi can only know the date if he has a unique day, the two are 17 and 18. So if Josef says Roi doesnt know then the months corresponding to these dates are eliminated
Jan 14,15,18 and Feb 16,17,13,15 are gone

So we are left with March 13,15 and Dec 13,14,16 meaning Roi has either a 13,14,15 or 16. It cannot be 13 because there is not enough information. That leaves Dec 14 and 16 and March 15. It cannot be either December date because there is not enough information so the answer has to be March 15

Correct

Answer is March 15th.

Assume Joseph knows the month. Joseph says "I know Roi doesn't know". Joseph knows that Roi only knows the day. The only way Roi could know the birthday is if Roi knew that it was 17th or 18th (as there is only one month for those feb and jan respectively). So for Joseph to know that Roi doesn't know, we know that Joseph must know that the month isn't Jan or Feb.

Hence it can only be March 13th, 15th, Dec 13th, 14th or 16th.

Roi says "I didn't know, but now I know" because he didn't know before if it was Jan or Feb or not, but now he knows it's not Jan or feb.

We now know it's not March 13th or Dec 13th, because otherwise Roi wouldn't know if it was March or Dec.

hence it can only be march 15th, dec 14th or dec 16th.

Josef then says "I now know too"

Thus it can only be March 15th, because Josef knows the month, and if he knew it was dec, he still wouldn't know if it was dec 14th or 16th, but as he knows, he must know it's march and there is only one march left march 15th.

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