Ok, I have received a couple of emails telling me that the solution is wrong and Cheryl's birthday is not Jul 16. It should be Aug 17.
I pondered for some time and concluded that for this question, some people get a different answer because they were looking at it from a different perspective. And it is flawed if we ONLY try to find out the dates by calculating backwards.
We should be finding the answer from first parties' perspective as well!
Let's first eliminate the impossible answers from the 2 statements given by A and B.
May and June are definitely out because they contain unique date and A would not make the first statement.
"14" is definitely out because both July and August contain "14" and B would not be able to make his statement after A's.
So, in conclusion, the answer(s)
can only, possibly be July 16, August 15 and August 17.
Let me show you how I obtained the answer from first parties' perspective.
A, B and C are classmates. A and B both were in love with C because C was extremely attractive.
So one day, A and B decided to ask C out for a date. C posted a question to them.
C: I will go out on a date with the person who can guess my birthday first. However, I will only tell A the "month" and B the "date".
C then wrote down 10 possibilities.
May 15
|
May 16
|
May 19
|
June 17
|
June 18
|
|
July 14
|
July 16
|
|
August 14
|
August 15
|
August 17
|
For answer JUL 16:
C tells A "July"
C tells B "16"
In A's mind:
May 15
|
May 16
|
May 19
|
June 17
|
June 18
|
|
July 14
|
July 16
|
|
August 14
|
August 15
|
August 17
|
In B's mind:
May 15
|
May 16
|
May 19
|
June 17
|
June 18
|
|
July 14
|
July 16
|
|
August 14
|
August 15
|
August 17
|
There was then an awkward silence.
However, with B's silence, A assumed that C had not been told B "18" or "19" because the two dates were unique and B would have sounded out the correct answer. So B should have either 14, 15, 16 or 17.
C: How is it? Any good guess?
A: I don't know when your birthday is, but I know that B does not know too.
On hearing this, B concluded that May and June were both out because A was very certain that he (B) was not given "18" or "19"! May 16 had been eliminated!
B: At first I don't know when C's birthday is, but I know now.
On hearing this statement, A then realised his mistake. He (A) had given B the clue that May and June were impossible and only July and August were possible months! Since B has a conclusion, the date cannot be 14 because B would not be able to conclude with 2 possibilities (July 14 and August 14).
C must have told B "16"!
A: Then I also know when Cheryl's birthday is.
If the answer was AUG 17:
C tells A "August"
C tells B "17"
In A's mind:
May 15
|
May 16
|
May 19
|
June 17
|
June 18
|
|
July 14
|
July 16
|
|
August 14
|
August 15
|
August 17
|
In B's mind:
May 15
|
May 16
|
May 19
|
June 17
|
June 18
|
|
July 14
|
July 16
|
|
August 14
|
August 15
|
August 17
|
There was then an awkward silence.
However,
with B's silence, A assumed that C had not told B "18" or "19" because
the two dates were unique and B would have sounded out the correct
answer. So B should have either 14, 15, 16 or 17.
C: How is it? Any good guess?
A: I don't know when your birthday is, but I know that B does not know too.
On
hearing this, B concluded that May and June were both out because A was
very certain that he (B) was not given "18" or "19"! June 17 had been
eliminated!
B: At first I don't know when C's birthday is, but I know now.
On
hearing this statement, A then realised his mistake. He (A) had given B
the clue that May and June were impossible and only July and August
were possible months! The date cannot be 14 because B would not be able
to conclude with 2 possibilities (July 14 and August 14). C must have
told B either "15" or "17"! And A cannot conclude because he did not know if it was Aug 15 or Aug 17!
A CANNOT CONCLUDE AT ALL!
So now you see, if we re-enacted the situation then and from first parties' perspective, we realised that August 17 CANNOT BE THE ANSWER! Because A simply would not know if C's birthday falls on Aug 15 or Aug 17, based on B's statement!
Regardless, B scored a point and had the chance to date C. And they probably lived happily ever after.
Tags: when is cheryl's birthday, albert, bernard, cheryl, solution, clue, answer, why Aug 17 is not the answer, Jul 16
