#cherylsbirthday - are you smarter than a 5th grader (in Singapore)? (1 Viewer)

[CletiusMaximus]

"Singapore math" (as it is called) is highly regarded by many, focusing on conceptual understanding of math concepts rather than rote memorization of processes.

Here's the test question making the rounds on twitter under the hashtag #cherylsbirthday - from a 5th grade exam in Singapore:

http://cdn.theatlantic.com/assets/media/img/posts/2015/04/11155125_385751114937109_7956840234831081160_o/c2431317d.jpg

I interrupted my workday to figure it out, then checked and confirmed I was correct, but I think I got the right answer somewhat by luck rather than rigorous proof. My 4th grader loves math, but I don't think he would know how to even begin solving this.

 

[shady inc]

i got this wrong. way too hard

 

[Simon Shepherd]

I think the main "trick" here is how poorly worded the question is.

Would be clearer as "Cheryl then tells Albert the month and Bernard the day in the month". Albert's statement of "I don't know when Cheryl's birthday is" is also redundant as no month has a single option in it.

I'll confess to having cheating on it though ;-)

 

[AcerFC]

Pretty sure this is beyond me

 

[TenTimes]

Maybe a technicality, but this would be a logic problem, not math, right?

I don't think 'logic' is a class you have to take to graduate high school, but probably could/should be.

 

[HughHoney]

I thought it was May 19th but that's wrong.

 

[TheIronSheik]

Cheryl is a liar. No one should trust that ##### to tell anyone what her actual birthday is.

 

[CletiusMaximus]

Albert's statement of "I don't know when Cheryl's birthday is" is also redundant as no month has a single option in it.

True, but Albert's statement gets one thinking in the right direction in terms of solving it.

Disagree with Sheik that Cheryl is necessarily a liar. I would use a much less flattering word to describer her.

 

[Gawain]

Reminded me a lot more of an LSAT logic problem than a math problem

 

[bostonfred]

I thought it was May 19th but that's wrong.

Neither one of them knew the answer without hearing from the other. so it can't be any day that's not repeated because knowing the day is the 19th tells you the month

it also can't be any month where eliminating a day that's not repeated eliminates the month. So if the days in may are the 14th and the 19th, knowing the month wouldn't be enough to tell you which day, but knowing its the 19th would. If the first kid knows it's may, then he can't know that the second kid doesn't know which day in may.

So it has to be a month that has two or more days which are repeated, and it has to be a day that's repeated.

but for the second kid to eliminate other options based on the first kid's response, it also has to be a day that's repeated in a month that doesn't meet those criteria. And it can't be a month with too many choices because you can't eliminate enough.

 

[bostonfred]

Albert's statement of "I don't know when Cheryl's birthday is" is also redundant as no month has a single option in it.

True, but Albert's statement gets one thinking in the right direction in terms of solving it.

The second kid needs to actually hear the first kid say that to solve the problem

 

[Bruce Dickinson]

"Singapore math" (as it is called) is highly regarded by many, focusing on conceptual understanding of math concepts rather than rote memorization of processes.

Here's the test question making the rounds on twitter under the hashtag #cherylsbirthday - from a 5th grade exam in Singapore:

http://cdn.theatlantic.com/assets/media/img/posts/2015/04/11155125_385751114937109_7956840234831081160_o/c2431317d.jpg

I interrupted my workday to figure it out, then checked and confirmed I was correct, but I think I got the right answer somewhat by luck rather than rigorous proof. My 4th grader loves math, but I don't think he would know how to even begin solving this.

The question is actually from a math competition for 14-year-olds in Singapore. It was intended to be difficult for 8th graders who are good at math.

 

[spOOfy]

i have not looked this up, but it seems easy enough...

Spoiler

Albert says he knows Bernard doesn't know the right answer. That rules out May and June. Those are ruled out because they have unique numbers - 18 & 19. That is the only way Albert can say that with any confidence. That leaves the options of July & August. Bernard says he knows the answer now. That rules out July 14 & August 14 because he wouldn't know the answer now if it was 1 of those. That leaves 3 dates. July 16, August 15 & August 17. Bernard then says he knows the answer. The only way he could know would be if it is July 16 because August has 2 dates.

So the answer = July 16

correct?

 

[Sinn Fein]

Spoiler

First guy knows the Month, but knows the date is not unique - so that rules out May and June

July and August are possibilities, but that seems like it leaves July 16, August 15, and August 17 as possibilities

ETA- seeing Spoof's answer, I forgot to include the info from when the first guy responded again....

 

[CletiusMaximus]

"Singapore math" (as it is called) is highly regarded by many, focusing on conceptual understanding of math concepts rather than rote memorization of processes.

Here's the test question making the rounds on twitter under the hashtag #cherylsbirthday - from a 5th grade exam in Singapore:

http://cdn.theatlantic.com/assets/media/img/posts/2015/04/11155125_385751114937109_7956840234831081160_o/c2431317d.jpg

I interrupted my workday to figure it out, then checked and confirmed I was correct, but I think I got the right answer somewhat by luck rather than rigorous proof. My 4th grader loves math, but I don't think he would know how to even begin solving this.

The question is actually from a math competition for 14-year-olds in Singapore. It was intended to be difficult for 8th graders who are good at math.

Yes - I see that correction now in the article I took this from. It went viral as taken from a "P5" test, which would be essentially 5th grade in the US, but was later confirmed to come from a math competition for kids approx. in 9th grade.

 

[Officer Pete Malloy]

I know one little b itch that isn't getting jack #### for her birthday.

 

[chet]

GOT IT

 

[TheIronSheik]

If I'm walking with Cheryl, and I say, "Hey, Cheryl, when's your birthday?" And her response is "I'll tell you the date and your friend the month and you two can figure it out", Cheryl and I are no longer friends and I don't care when her birthday is. I ain't got time for #####es playing mind games on me. My friend can have that loon.

 

[Notorious T.R.E.]

i have not looked this up, but it seems easy enough...

Spoiler

Albert says he knows Bernard doesn't know the right answer. That rules out May and June. Those are ruled out because they have unique numbers - 18 & 19. That is the only way Albert can say that with any confidence. That leaves the options of July & August. Bernard says he knows the answer now. That rules out July 14 & August 14 because he wouldn't know the answer now if it was 1 of those. That leaves 3 dates. July 16, August 15 & August 17. Bernard then says he knows the answer. The only way he could know would be if it is July 16 because August has 2 dates.

So the answer = July 16

Spoiler

correct?

 

[elbowrm]

Apparently itwas 10th graders, not 5th - not that that makes that much different.

 

[Sinn Fein]

Actually I don't follow Spoofy's answer

Spoiler

First sentence eliminates May and June due to unique "dates" 18 and 19

Second sentence tells us that Benard knows the date which allows him to know which of July 16, August 15, or August 17 is - since those are now unique dates

How does Albert now "know" the birthdate? It could be August 15 or 17th - based on the information we have now. Albert can't "know" the exact date without more information

 

[Spin]

Got it using excel.

Spoiler

http://puu.sh/hf0qM/dc73ccd617.png

 

[Sinn Fein]

Actually I don't follow Spoofy's answer

Spoiler

First sentence eliminates May and June due to unique "dates" 18 and 19

Second sentence tells us that Benard knows the date which allows him to know which of July 16, August 15, or August 17 is - since those are now unique dates

How does Albert now "know" the birthdate? It could be August 15 or 17th - based on the information we have now. Albert can't "know" the exact date without more information

meh - nevermind - he knew the month, so it had to be only one answer...

 

[Spin]

Actually I don't follow Spoofy's answer

Spoiler

First sentence eliminates May and June due to unique "dates" 18 and 19

Second sentence tells us that Benard knows the date which allows him to know which of July 16, August 15, or August 17 is - since those are now unique dates

How does Albert now "know" the birthdate? It could be August 15 or 17th - based on the information we have now. Albert can't "know" the exact date without more information

Spoiler

Albert was given the month. He already knew it was July, just needed to figure out the date.

 

[spOOfy]

Actually I don't follow Spoofy's answer

Spoiler

First sentence eliminates May and June due to unique "dates" 18 and 19

Second sentence tells us that Benard knows the date which allows him to know which of July 16, August 15, or August 17 is - since those are now unique dates

How does Albert now "know" the birthdate? It could be August 15 or 17th - based on the information we have now. Albert can't "know" the exact date without more information

Spoiler

Albert was given the month. He already knew it was July, just needed to figure out the date.

exactly

 

[bigbottom]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

 

[bigbottom]

GOT IT

I'm glad you figured it out. Your initial response would have gotten you booted from whatever that brainiac society is that you belong to.

 

[chet]

GOT IT

I'm glad you figured it out. Your initial response would have gotten you booted from whatever that brainiac society is that you belong to.

My original question, although tongue in cheek, actually led me to the solution.

 

[IvanKaramazov]

Got it. Spoofy is right.

 

[Parrothead]

3.14

 

[Van Dyman]

Oddly enough, I figured out cheryl's birthday without much problem but couldn't for the life of me figure out how to do a "spoiler alert" thingy.

 

[Van Dyman]

Oddly enough, I figured out cheryl's birthday without much problem but couldn't for the life of me figure out how to do a "spoiler alert" thingy.

Spoiler

Ah, ok.

 

[TheIronSheik]

I think I made a mistake. I figured out Albert's birthday by accident.

 

[AcerFC]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

This is the first time I got it

I couldnt follow Spoofy and his math logic

 

[Van Dyman]

Albert and Bernard want to bone Cheryl. Bernard is a physical trainer. How old is Albert?

Hint

 

[Bull Dozier]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Question based on the answers:

Spoiler

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

 

[bigbottom]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Spoiler

Question based on the answers:

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

Spoiler

Albert knows definitively that Bernard doesn't know the birthday. Albert wouldn't be able to say that if the month was May or June - because if it were May or June, Bernard could have one of the unique days and therefore know the birthday. In other words, it can't be a month where it is possible that Bernard would know the birthday.

 

[Officer Pete Malloy]

Anybody say "golf holes" yet?

 

[matttyl]

This is why Cheryl didn't get any birthday presents.

 

[Bull Dozier]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Spoiler

Question based on the answers:

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

Spoiler

Albert knows definitively that Bernard doesn't know the birthday. Albert wouldn't be able to say that if the month was May or June - because if it were May or June, Bernard could have one of the unique days and therefore know the birthday. In other words, it can't be a month where it is possible that Bernard would know the birthday.

Got it now.

 

[Fennis]

this was making the rounds at my work a couple days ago. fun little logic puzzle.

 

[GregR]

I keep getting "42".

 

[Jayrok]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Spoiler

Question based on the answers:

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

Spoiler

Albert knows definitively that Bernard doesn't know the birthday. Albert wouldn't be able to say that if the month was May or June - because if it were May or June, Bernard could have one of the unique days and therefore know the birthday. In other words, it can't be a month where it is possible that Bernard would know the birthday.

What if Bernard was told the day was 17? If May and June are ruled out, then he would know right away that it was August 17 since July doesn't have 17 as a choice. Same goes for August 15.

 

[Evilgrin 72]

9th grade, I get this question right in about 14 seconds, then take Cheryl into the woods behind the school and fingerblast her while smoking a Marlboro Red.

 

[Spin]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Spoiler

Question based on the answers:

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

Spoiler

Albert knows definitively that Bernard doesn't know the birthday. Albert wouldn't be able to say that if the month was May or June - because if it were May or June, Bernard could have one of the unique days and therefore know the birthday. In other words, it can't be a month where it is possible that Bernard would know the birthday.

What if Bernard was told the day was 17? If May and June are ruled out, then he would know right away that it was August 17 since July doesn't have 17 as a choice. Same goes for August 15.

But then Albert would have been told "August" and he still wouldn't know if it was the 15th or the 17th of August.

 

[GregR]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Spoiler

Question based on the answers:

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

Spoiler

Albert knows definitively that Bernard doesn't know the birthday. Albert wouldn't be able to say that if the month was May or June - because if it were May or June, Bernard could have one of the unique days and therefore know the birthday. In other words, it can't be a month where it is possible that Bernard would know the birthday.

What if Bernard was told the day was 17? If May and June are ruled out, then he would know right away that it was August 17 since July doesn't have 17 as a choice. Same goes for August 15.

If Bernard was told 17, then Albert's final statement would have been, "I still cannot tell the date of the birthday". Because Albert wouldn't be able to tell between August 15th and 17th.

Since Albert didn't say that, instead saying he knows the birthday, we know it had to be July 16.

 

[bigbottom]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Spoiler

Question based on the answers:

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

Spoiler

Albert knows definitively that Bernard doesn't know the birthday. Albert wouldn't be able to say that if the month was May or June - because if it were May or June, Bernard could have one of the unique days and therefore know the birthday. In other words, it can't be a month where it is possible that Bernard would know the birthday.

What if Bernard was told the day was 17? If May and June are ruled out, then he would know right away that it was August 17 since July doesn't have 17 as a choice. Same goes for August 15.

If it was either of those dates in August, then Albert wouldn't know the date per clue #3.

Edit: asked and answered

 

[stbugs]

Maybe a technicality, but this would be a logic problem, not math, right?

I don't think 'logic' is a class you have to take to graduate high school, but probably could/should be.

Yeah, it wasn't really that hard if you wrote it down and removed the values that were eliminated, but definitely a logic problem. This has nothing to do with math IMHO.

 

[stbugs]

I'm sure this has already bee solved multiple times while I've been noodling the problem, but here's my solution anyway:

Spoiler

First clue: Albert knows the month, and knows that Bernard doesn't know the birthday, which means that the day has to appear more than once (and any month with a unique day would be ruled out). That rules out both May and June. So Albert is given either July or August.

Second clue: Bernard now knows the birthday after realizing that the first clue narrows it down to July or August. As such, the date cannot be either July 14 or August 14. That leaves July 16, August 15 or August 17. Bernard knows the day, so he now knows what the birthday is.

Third clue: Since Albert now knows what the birthday is, it must be during July, since August would leave two choices.

Cheryl's birthday is July 16.

Spoiler

Question based on the answers:

Why does the first clue rule out entire months, instead of just those individual days in the month that are unique? Albert says he knows Bernard doesn't know, so why does that rule out the entire month of May, and not just May 19?

I got August 17 FTR, just admitting I got this wrong, but I am still convinced I could be right until I understand the answer to the above.

Spoiler

Albert knows definitively that Bernard doesn't know the birthday. Albert wouldn't be able to say that if the month was May or June - because if it were May or June, Bernard could have one of the unique days and therefore know the birthday. In other words, it can't be a month where it is possible that Bernard would know the birthday.

What if Bernard was told the day was 17? If May and June are ruled out, then he would know right away that it was August 17 since July doesn't have 17 as a choice. Same goes for August 15.

If it was either of those dates in August, then Albert wouldn't know the date per clue #3.

Edit: asked and answered

Yep, you guys did it properly.

 

[Bruce Dickinson]

Maybe a technicality, but this would be a logic problem, not math, right?

I don't think 'logic' is a class you have to take to graduate high school, but probably could/should be.

Yeah, it wasn't really that hard if you wrote it down and removed the values that were eliminated, but definitely a logic problem. This has nothing to do with math IMHO.

Logic is the source material for math. Understanding logic is essential for understanding higher levels of math.