## Monday, April 13, 2015

### P5 Maths question - When is Cheryl's birthday?

PHOTO: Here’s a supposedly P5 logic question going viral on Facebook that will melt your brain
Posted by Jonathan Lim on 12 April 2015
http://2.bp.blogspot.com/-ZsnMS9hk6do/VStHFUGPHeI/AAAAAAAAfzY/xyMdPn1_zLw/s1600/trick-question.jpg

This logic was shared by a MediaCorp Channel 8 presenter Kenneth (江坚文 Jiāng jiān wén) [https://www.facebook.com/kennethjianwenz/photos/a.173663129479243.1073741827.167504136761809/385751114937109/?type=1&theater] on Facebook on Saturday 11 Apr.

In his Facebook post he said “This question causes a debate with my wife …. and its a P5 question. #trickquestion” [https://www.facebook.com/hashtag/trickquestion]

It is not known whether this is really a Primary 5 question or if it came from Singapore, but more importantly, can you solve the question?

Table posted by Stephanie Ng Wan Jing
Texts (explanations) posted by Jun Cai

PHOTO: P5 logic question
Posted by Jonathan Lim on 12 April 2015
http://2.bp.blogspot.com/-ZsnMS9hk6do/VStHFUGPHeI/AAAAAAAAfzY/xyMdPn1_zLw/s1600/trick-question.jpg

July 16 is the only possible answer
1) Albert: I don't know when Cheryl's birthday is, but l know that Bernard does not know too.

A knows B confirm don't know the date, which means A wasn't told May or June because May and June both have an unique identifier (5/19 and 6/18) ie. If B was told 19 or 18 the entire Condition 1 is wrong.

2) Bernard: At first I did not know when Cheryl's birthday is, but I know now.
We are left with July to August. B is now confident of his answer, so his Date must be unique, thus eliminating 7/14 and 8/14.

3) Albert: Then I also know when Cheryl's birthday is.
A says he now knows the answer, because July is the only unique (There are two August dates, left)