Brainteasers

Newsletter November 2020

Sarah's birthday is one of 10 possible dates:

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


Sarah tells the month to Albert and the day to Bernard.


Albert: I don’t know the birthday, but I know Bernard doesn’t know either.
Bernard: I didn’t know at first, but now I do know.
Albert: Now I also know Sarah’s birthday.


When is Sarah’s birthday?

Hover cursor here to reveal answer

If Albert knows that Bernard doesn’t know when the birthday is, then the birthday can’t be on May 19th or June 18th as there is only one 19th and one 18th. Also, Albert must know that the birthday can’t be on these dates, so May and June are completely ruled out.

If Bernard can deduce when the birthday is after Albert’s comment, then the birthday can’t be on 14th.


The remaining possibilities are July 16, August 15, and August 17.


Finally, if Albert figures out when the birthday is after Bernard’s comment, then the date must be July 16.

Previous Newsletter

How can you take 2 from 5 to leave 4?

Hover cursor here to reveal answer

Write  5 as F-I-V-E and take away the F and E and you are left with IV which is roman numeral for 4

Adam Caar

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