when is cheryl's birthday?

[TightEnd]

this test for 14 year olds uses logical reasoning and aims to sift out the most intelligent

It was set in the Singapore and Asian Schools Maths Olympiads (SASMO)

what is the answer?



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 don’t 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?

[Graham C]

There's enough info there to get it?

[TightEnd]

There's enough info there to get it?

Yes

but it is very very difficult indeed

(i couldn't get it, but have answer...i can put people on the right track if you collectively want to solve it)

[celtic]

June 18?

[RED-DOG]

Appleby time?

[TightEnd]

i'm going to need some reasoning...but both the guesses above are incorrect

[RED-DOG]

i'm going to need some reasoning...but both the guesses above are incorrect

A lot of the older Gypsies didn't know exactly what date they were born on, so if asked they would usually reply with something like:

Appleby time.

In the strawberries.

Bonfire night.

Doncaster races.

Etc etc

[edgascoigne]

this test for 14 year olds uses logical reasoning and aims to sift out the most intelligent

It was set in the Singapore and Asian Schools Maths Olympiads (SASMO)

what is the answer?



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 don’t 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?

The fact Albert (month) knows Bernard (date) does not know rules out the single dates - May 19 & June 18. All other dates repeat.

Bernard now knows the date (the bit he knows) must be repeated, but also that it cannot be June 17 as Albert has ruled out June 18 and still doesn't know.

So Bernard is left with, effectively, a sudoku with two dates in May, two in July and THREE in August. The 14/15/16 cannot be know, leaving only:

AUGUST 17th









I think?!

[TightEnd]

good start, right sort of logical reasoning

wrong answer

[Longy]

I won't give it away as I saw it yesterday but the 3 us discussing it, came up with 3 different answers! Easy it is not.

[edgascoigne]

good start, right sort of logical reasoning

wrong answer

Feck.

[KarmaDope]

If Albert is sure that Bernard doesnt know then it completely rules out May or June because they are the only months with date numbers that do not repeat.

This leaves us with July or August. Bernard now knows the date but still only has the number. This rules out July 14 and August 14 because he wouldnt be sure if it was the 14th.

Albert now says that he knows. In that case it has to be the remaining July date as if it was August he couldnt be 100% sure so I'm going to guess that it is July 16th.

[TightEnd]

If Albert is sure that Bernard doesnt know then it completely rules out May or June because they are the only months with date numbers that do not repeat.

This leaves us with July or August. Bernard now knows the date but still only has the number. This rules out July 14 and August 14 because he wouldnt be sure if it was the 14th.

Albert now says that he knows. In that case it has to be the remaining July date as if it was August he couldnt be 100% sure so I'm going to guess that it is July 16th.

very good

the full article with expansive reasoning is at

http://www.dailymail.co.uk/sciencetech/article-3037266/The-maths-problem-set-Singapore-teenagers-left-people-world-stumped.html

[EvilPie]

good start, right sort of logical reasoning

wrong answer

Feck.

Once you've ruled out June 17th you've can also rule out August 17th by the same logic as step 1.......

[Tal]

I've gone much the same way as Ed to start with.

You've excluded two of the May and June dates, which means that the other one has to fall, too, if Albert knows for sure that Bernard doesn't know. Once you've got May out of the way, you know it's July or August.


I'm trying to prepare for a chess match, here! I'll come back tonight once I have more!