Cheryl's Birthday
This time I present the famous problem recently raised at the Olympics in math Asia and Singapore from 2015 to kids in high school, which has already converted in viral on social networks, which is a sign of the interest that may raise a good conundrum in the general public. In the illustrations of this riddle I pay tribute to the great actors of an excellent movie that I recommend: "Philadelphia Story (Philadelphia Story, 1940)".
" Albert and Bernard have recently met Cheryl, and both want to find out when it's your birthday. Cheryl gives them a list of 10 possible dates:
- May 15
- June 17
- July 14
- August 14
Cheryl tells Albert and Bernard separate the month and day of your birthday respectively.
Albert: I do not know when Cheryl's birthday, but I know Bernard does not know.
So when is the birthday of Cheryl ?.
SOLUTION:
When reading the problem statement , one can stay stunned and go blank. However, anyone who has a little experience in addressing problems of logic and learn how to use the basic logic, perhaps for being a faithful follower of this blogor my channel YouTube ,; ), can reach the solution, since no it is an extremely difficult problem; however you have to combine various mental processes, so it is an excellent exercise for the brain.
Let the matter:
We have 10 dates, with 6 numbers we see that some numbers are repeated except 18 and 19. Albert knows only the month of the birthday of Cheryl, and Bernard only knows the number.
In the first comment of Albert, who remember who knows the birthday month Cheryl tells us that you do not know your birthday but know that Bernard (who knows the number but not the month) does not know either. Bernard could only know the birthday if he knew the number was 18 or 19, which are the numbers that are not repeated in the list given by Cheryl. For Albert knew that Bernard does not know the birthday, it should fall into a different month to appear with the numbers 18 or 19. We are then 5 dates, July and August:
- 14 and 16 July
With commentary by Bernard indicating that after the comment of Albert already know the date of the birthday, we infer that the number of the date that he knows should not be repeated in the list above, then, of those five dates must discard dates there are any number to repeat, staying only 3 dates:
- July 16
Now in the second comment of Albert (who knows Cheryl's birthday month) it says that from the comment of Bernard and also knows that date is the birthday of Cheryl. As he knows the month, the birthday should be the July 16 , as if it were the birthday in August would have no way of knowing if the birthday is on 15 or 17 August.
To return to the main page click HERE.
- May 15
- May 16
- May 19
- June 17
- June 18
- July 14
- July 16
- August 14
- August 15
- August 17
SOLUTION:
Let the matter:
We have 10 dates, with 6 numbers we see that some numbers are repeated except 18 and 19. Albert knows only the month of the birthday of Cheryl, and Bernard only knows the number.
In the first comment of Albert, who remember who knows the birthday month Cheryl tells us that you do not know your birthday but know that Bernard (who knows the number but not the month) does not know either. Bernard could only know the birthday if he knew the number was 18 or 19, which are the numbers that are not repeated in the list given by Cheryl. For Albert knew that Bernard does not know the birthday, it should fall into a different month to appear with the numbers 18 or 19. We are then 5 dates, July and August:
- 14 and 16 July
- 14, 15 and 17 August
With commentary by Bernard indicating that after the comment of Albert already know the date of the birthday, we infer that the number of the date that he knows should not be repeated in the list above, then, of those five dates must discard dates there are any number to repeat, staying only 3 dates:
- July 16
- 15 August 17
Now in the second comment of Albert (who knows Cheryl's birthday month) it says that from the comment of Bernard and also knows that date is the birthday of Cheryl. As he knows the month, the birthday should be the July 16 , as if it were the birthday in August would have no way of knowing if the birthday is on 15 or 17 August.
To return to the main page click HERE.
