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Last active May 27, 2020 18:52
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Cheryl's Birthday (aka Birthday problem)
https://en.wikipedia.org/wiki/Cheryl%27s_Birthday
A knows the month
B knows the day
The candidates are:
may 15 16 19
june 17 18
july 14 16
august 14 15 17
A asserts B doesn't know the date. He can only say that with certainty if the month
(that A knows) doesn't contain unique days. That rules out may (which contains 19)
and june (which contains 18).
B, realizing the fact above, reduces the list to:
july 14 16
august 14 15 17
and says he knows the date. That implies the the day is NOT 14 (after all, he could not
know the month in that case)
With that in mind, A can reduce the list to:
july 16
august 15 17
And now A says he knows the day. But that means the month can't be august (because it
has two days), hence we rule out august, and the date must be July 16!
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