Skip to main content

When’s your birthday? ~ A Singapore Math Problem With The Solution

Question:

Albert and Bernard just met Cheryl. “When’s your birthday?” Albert asked Cheryl.
Cheryl thought a second and said, “I’m not going to tell you, but I’ll give you some clues.” She wrote down a list of 10 dates:
May 15 — May 16 — May 19
June 17 — June 18
July 14 — July 16
August 14 — August 15 — August 17
“My birthday is one of these,” she said.
Then Cheryl whispered in Albert’s ear the month — and only the month — of her birthday. To Bernard, she whispered the day, and only the day. 
“Can you figure it out now?” she asked Albert.
Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either.
Bernard: I didn’t know originally, but now I do.
Albert: Well, now I know, too!
When is Cheryl’s birthday?

 Solution:

First Clue:
I don’t know when your birthday is, but I know Bernard doesn’t know, either.
How does Albert know that Bernard does not know?  Albert has no clue that Day is not 18th or 19th.  This means the month cannot be May or June.

Second Clue:
I didn’t know originally, but now I do.
After knowing Albert’s statement, Bernard now also knows that Cheryl’s birthday is not in May or June.  With this information, Bernard now knows the exact birthday.

This means it won't be 14th because there would still be two possibilities: July 14 and Aug. 14.

Last Clue:
Well, now I know too!
With 3 choices left (July 16th, August 15th, August 17th), Albert knows the birthday as well.  It is the same logical process again.  If Cheryl had told him August, Albert would not know the birthday.  It must be July then.

The answer is July 16.

 Fun?

Comments

Popular posts from this blog

Melbourne Earthquake?

Image extracted from The Age Was it an earthquake just now? Updates:  4.5 earthquake rocked south Victoria (near Moe) at July 20, 2012 7:11pm News Update:  News.com.au  or ninemsn  or The Age .

Box Hill High School Zone

Parents are struggling with moving into a good reputable school for their kid(s). Here is the zone for Box Hill High School.   Good luck with house hunting!

Brivis Heater - Error Code #69

My Brivis HX23 Heater broke down this week.  H01 Code #69.  This code was not even listed in the User Manual!  Searching around the web and here is what I found: Secret Reset: This is not in the User Manual.  On the networker controller press FUNCTION > 1 > 4 keys and the screen will go blank and come back on after 30 seconds, this has rebooted the main PCB and reset the fault code. Use this with caution as too much gas build up could cause an explosion. Use it only after the unit has enough time to air out the gas build up. What is Code #69 or any other unpublished codes?  The best way to find out is to open up the heater unit.   There is a little LCD on the PCB showing what error exactly is. (Note: Opening up the unit may void your warranty.)