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This is a famous Singaporean Logic Problem.
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Albert and Bernard just become friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates:
•May 15, May 16, May 19
•June 17, June 18
•July 14, July 16
•August 14, August 15, August 17
Cheryl then tells Albert and Bernard separately the month and day of her birthday respectively.
Albert: I don't know when Cheryl's birthday is, but I know for sure that even Bernard doesn't know it.
Bernard: At first I didn't know when Cheryl's birthday is, but I know now it.
Albert: Now I also know when Cheryl's birthday is.
So when is Cheryl's birthday?
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I got the solution already, but I don't understand a thing!!! Please help me...
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- Since Albert does not know Cheryl's birthday on the first time, the month have at least 2 days in contention (doesn't really help).
- Albert knows that Bernard doesn't know it, so all of the days in that month (which Albert knows) have alternatives. This must be July or August.
- Now Bernard knows the birthday, just from knowing that it is July OR August, and from the day. Therefore the birthday cannot be July/August 14, or Bernard would never have figured out.
- If Albert has got August initially, at this point he wouldn't be able to tell whether if it was August 15 or 17. Therefore Albert must have received July at first, and so the birthday was July 16.
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Thanks... it really helped me.
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