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Current time:0:00Total duration:9:29

Video transcript

so this is a fund adducted reasoning problem that's being forwarded around the internet several people have actually forwarded it to me and so I thought it would be a fun thing to work through the grammar isn't perfect but the problem is a fun one so it says Albert and Bernard justbe could say became friends with Cheryl and they wanted to know when her birthday is Cheryl gives them a list of ten possible dates and we have our ten dates here Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively just to make sure we're parsing that sentence correctly she gives Albert she tells Albert the month the month of her birthday and she gives Bernard she gives Bernard the day of her birthday and I think we it's fair to assume that Albert knows he's been given the month and he knows that Bernards been given the day and Bernards been given the day and he knows that Albert has been given the month and we should also soom that they are good logicians that they aren't they're going to do very logical things and then we have this dialogue Albert says I don't know when Cheryl's birthday is but I know that Bernard does not know - that's fascinating and then Bernard says at first I didn't know when Cheryl's birthday is but now I know but now I know fascinating and then Albert says then I also know when Cheryl's birthday is so when is Cheryl's birthday so like always pause the video see if you can work through this on your own it's a fun problem to tackle and I'll give you a little bit of a hint diagram out write the information that each that Albert knows and how does the world look from Albert's point of view and then write out how the world looks from Bernards point of view and then tell us and then think about what these what you can deduce from these sentences so I'm assuming you've had a go at it so let's just do what I just said let's let's diagram out the world from each of their points of view so let's first from Albert's point of view at a four Albert so he's been given the month so he's been given either May and if it's May he says well then if it's May it's going to be the actual day is going to be 15 16 or 7 15 16 or 19 15 16 or 19th he was told you or he was told the month June and if he was told the month of June then he says well I don't know if the date is 17 or 18 17 or 18 if he was told the month of July if he was told July he'd say well I don't know whether the date is for the 14th or the 16th and if he was told them onto August if he was told the month August he'd be thinking to himself well the dates either going to be the 14th the 15th or the 17th now let's think about the world from Bernards point of view from Bernards point of view he's been given the date so if he was been given the date the 14th if he was told the 14th then he would say all right let's see May is not an option June wouldn't be an option July would be an option and August would be an option because there's just alive 14 and there's an August 14 there's no May 14th or June 14th so he would say well I'm not sure if he was told it was the 14th then it's either going to be July or August if he was told the 15th then it would be lets see May would be an option there's a May 15th year or August would be an option there's no June 15th of July 15th year so May May or autist if he was told the 16th let's see what are the options it could be May there's a May 16th as one of the options there's no June 16th there is a July 16th so he'd say maybe it's July and there's no August 16th either if he was told it's a 17th if he was told it's the 17th you could say well it's not gonna be May there's no May 17th there is a June 17th there is a June 17th there's no July 17th but there is an August 17th so there is an August there is I'll just write OGG for short there is an August 17th and then if he was told an 18th he would say let's see there's no May 18th there is a June 18th and I think that's the only 18th we have here so if he was told that the Day is the 18th then he would know he would know that the month is June and if he was told that the day is the 19th then up here the only 19th here is May 19th May May 19 so if Bernard Bernards interesting if he was told the 18th or the 19th then he would know what the month is so with this out of the way and if you're getting inspired I encourage you to pause the video again and see if you could work through these statements but now let's work through the statements so Albert says I don't know when Cheryl's birthday is which makes sense because no matter what month he's been given here there's multiple options for each of those months so it makes sense that he doesn't know when Cheryl's birthday is but I know I know that Bernard does not know too so how could he be so confident that Bernard does not know too well Albert has the same information that we have right over here and and Albert says well look if the date if Bernard was given an 18 or a 19 then he would he would know for sure that Bernard does know the date so the only way that he can feel confident the only way that he can feel confident that Bernard - the only way he could say I know that Bernard does not know - means that Albert knows that Bernard did not get an 18th or 19th so another way to translate this the statement is Bernard Bernard did not get an 18th or 19th Bernard not get 18th or or 19th now how can Albert be so confident that Bernard did not get an 18th or 19th well Albert knows Albert knows the month Albert knows the month and if you know if he's so confident that Bernard did not get an 18th or 19th that's because he knows it's not May or June if the month that Albert got was May then he would say well maybe he did get a 19th year in which I can't couldn't make this statement or if the month was June well then 18th would have been a possibility for what Bernard got and so he wouldn't have been able to make this statement here but because he says I know that Bernard does not know - that he the reason why he can say that is because he knows it's not May or June so we can rule out we can rule out May or oonh so we can rule out we can rule out the 18th the 19th year and we can also rule out May or June now from Barnard now from Bernards point of view we could say look Albert was the owners why he was able to make that statement is because Albert knows that the month is not May or June so let me rule out all of the Mays and June's so let me rule out June here let me rule out let me rule out May over here now I think you see where this is going now Bernard and once again at any point if you get so inspired please pause the video and continue on with the deductive reasoning that we have started all right now Bernard says at first I don't or I didn't know when Cheryl's birthday is which is fair because it wasn't the 18th or 19th which Albert are he knew but now I know so now I know so before he didn't know but now he knows so if now he knows what are the possibilities what are what are the possibilities going on here so he knows if it's if the if the date he was given is the 17th we've been able to then he could deduce that hey the month is now August if the date he was given was the 16th the month that he could now deduces July if the date he was given was the 15th he could deduce August but if the date he was given was 14th he still wouldn't know the month what the month is it could still be July or August so we could rule out the 14th because the 14th the date he was given to the 14th he still wouldn't know but he says now I do know so we could rule out the 14th and once again we're assuming Albert is hearing everything Albert and Bernard they both could kind of draw these diagrams out and so Albert could now say ok look if if he now knows then the date couldn't have been the 14th so then he rules out the 14th he rules out the 14th so the stuff in orange that I just did I got from this statement there and then Albert says then I also know when Cheryl's birthday is so from Albert's point of view the only way that he now knows what the birthday is if the month he was given was August he still has two options there but he says he now knows when Cheryl's birthday is so it can't be August because all the deductive reasoning we've just found done if he was given August then he would still not know whether whether it was the 15th or 16th but now that he knows it must be July because if he was given the month July there's only one option now for the date so therefore the date must be July July 16 Albert must have been given July Albert must have been given July and Bernard must have been given must have been given the 16th so Cheryl was born on July 16th lots of fun