~British Soul~ Posted April 14, 2015 Report Share Posted April 14, 2015 A TV presenter in Singapore has sparked an unlikely viral craze after posting a maths problem that was posed to the nation’s children. Initially confusing but very doable, it has sparked thousands of comments and shares – particularly when it was initially thought to have been posed to 10-year-olds. Could you solve it? (I will post the solution in a spoiler later) Link to comment Share on other sites More sharing options...
Trebuchet MS Posted April 14, 2015 Report Share Posted April 14, 2015 By the way, that's a Sec 3 Math Olympiad question - in other words, it (and others of its kind) is designed specifically for 15-year-olds who are incredibly adept at math. Normally not available in mainstream syllabus. [spoiler Anyway...]If Albert knows that Bernard doesn't know, that means he didn't get a month that corresponds to a number that appears only once on the chart (18 or 19). Which means he didn't get May or June. Now that Albert has indirectly mentioned he doesn't have May or June, Bernard knows for sure when the birthday is. This means Bernard's number is not repeated amongst July and August, so he got either 15, 16, or 17. If he had 14, he still wouldn't know as his options are either July or August 14. With this possibility added, we now have 3 possible dates: July 16, August 15, August 17. Since Albert now knows Cheryl's birthday after Bernard's statement, we can conclude his month has only 1 remaining possible number out of the three leftover dates. Otherwise he'd still be uncertain about Bernard's number. Cheryl's birthday is July 16.[/spoiler] Link to comment Share on other sites More sharing options...
Forest Fire Posted April 15, 2015 Report Share Posted April 15, 2015 I see two possibilities: [spoiler Either:] May 19 or June 18. They are the only two numerical dates that are not repeated; meaning that if it was Bernard knew the date and it was one of those he'd be able to tell from only the date when her birthday is. I don't see how Bernard knowing makes Albert know though, unless he told Albert (either the numerical date alone or the month and date); since knowing the month alone really can't definitively tell when her birthday is as far as I can see.[/spoiler] I'm gonna look at Gadj's spoiler to see if I'm right on (at least one of) my assumptions Okay. I see why I'm wrong. That actually makes a lot of sense. Link to comment Share on other sites More sharing options...
Tinkerer Posted April 15, 2015 Report Share Posted April 15, 2015 Huh... I'm not sure if this is right or not but... [spoiler Spoiler:] August 17. May 19 and June 18 were eliminated since the second guy didn't know the answer immediately. If June 17th was the answer, the first guy would've figured it out based on the month. At this point, every date except August 17 is repeated at least once. Second guy would have figured out that it couldn't have been June or first guy would've said he knew, therefore he figured out that his 17th must be August 17th.[/spoiler] Link to comment Share on other sites More sharing options...
Tentacruel Posted April 15, 2015 Report Share Posted April 15, 2015 This seems more like logic than math, but I took a crack at it. [spoiler=spoiler] A knows B can't know the day. The only way B could know is if the day was 18th or 19th. This must mean the month is July or August Knowing that, B knows the day. Because of this, it can't be 14. B could know the date is 15, 16, or 17, but A knows the date now because of this. If the date was in August, A would have no way of knowing, but if he knew the date was in July, and knew it couldn't be 14, he would know it's July 16th. [/spoiler] Yay it looks like I'm right. Link to comment Share on other sites More sharing options...
Tinkerer Posted April 15, 2015 Report Share Posted April 15, 2015 @ Tent or Gaj [spoiler Question...] How are you eliminating the other dates in May? That's the only thing I didn't understand...[/spoiler] Link to comment Share on other sites More sharing options...
Tentacruel Posted April 15, 2015 Report Share Posted April 15, 2015 @ Tent or Gaj [spoiler Question...] How are you eliminating the other dates in May? That's the only thing I didn't understand...[/spoiler] [spoiler Answer] We know that it can't be May or June at all. A knows that B can't know the date. The only way for B to know the date is if the date is the 18th or 19th, as there are only one of each. The only way for A to know that the date isn't the 18th or 19th is if the month he was given does not contain those dates. So we're down to July and August. [/spoiler] Link to comment Share on other sites More sharing options...
Tinkerer Posted April 15, 2015 Report Share Posted April 15, 2015 [spoiler Answer] We know that it can't be May or June at all. A knows that B can't know the date. The only way for B to know the date is if the date is the 18th or 19th, as there are only one of each. The only way for A to know that the date isn't the 18th or 19th is if the month he was given does not contain those dates. So we're down to July and August. [/spoiler][spoiler Hmm?] Sorry, I'm still kinda confused. What clue did A give that showed that he knew the... date... Wait... I think... Are we assuming that the statements are the only exchange of information? I think what got me confused was assuming B told A that he didn't know her birthday before those statements were said. Wow... That little assumption changed the entire outcome. Thanks for breaking it down though.[/spoiler] Link to comment Share on other sites More sharing options...
Tentacruel Posted April 15, 2015 Report Share Posted April 15, 2015 [spoiler Hmm?] Sorry, I'm still kinda confused. What clue did A give that showed that he knew the... date... Wait... I think... Are we assuming that the statements are the only exchange of information? I think what got me confused was assuming B told A that he didn't know before those statements were said. Wow... That little assumption changed the entire outcome. Thanks for breaking it down though.[/spoiler] Yup. It's a bit tricky. My conclusion was based on assuming that the statements are the only exchange of information between the two. Link to comment Share on other sites More sharing options...
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