#4. First understand the question!
Albert and Bernard just became 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 the day of her birthday respectively.
Albert: I do not know when Cheryl's birthday is, but I know that Bernard does not know too.
Bernard: At first I don't know when Cheryl's birthday is, but I know now.
Albert: Then I also know when Cheryl's birthday is.
So when is Cheryl's birthday?
#3. let's find the solution
First we need to figure out if Albert knows the month or the day. If he knows the day, then there is no chance that Bernard knows the birthday, so it must be that Albert knows the month. From the first statement, we know that Albert is sure that Bernard doesn't know the birthday, so May and June should be ruled out (the day 19 only appears in May and the day 18 only appears in June). In other words, if Albert had May or June, then he cannot be sure that Bernard doesn't know, since Bernard could have had 18 or 19.
Following that statement, Bernard knows that May and June are ruled out.
So, let's cut all the dates of May and June from the question.
Check further pages to find solution...
#2. Further exercise
Then, Bernard is able to know which month it is. So it must be either July 16, August 15 or August 17 (not 14th as then he can't know because there are two months with the same date).
Let's cut July and August 14 from the question.
#1. Final answer
Since Albert subsequently can also be sure of the date, he must know it's July. If it's August, he can't be sure as there is August 15 and 17.
So the answer is July 16.
