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Current time:0:00Total duration:6:49

Liar truth-teller brain teaser

Video transcript

this brain teaser is a bit of a classic and I'm sure a bunch of you heard it before but I wanted to include this in the brain teaser playlist that I'm making right now because this one is actually a good build-up and a bit of a nice warm-up session for a slightly harder brain teaser that deals with people telling the truth and lying so this one we could call it the D you know D truth teller and the liar truth teller and liar so we're on some type of adventure quest and we get to a fork in a road right so I have or let's say that there's a two doors let's make it that's even better so there's two doors and I've gotten it I guess I've gone to the end of my adventure quest and I know that my treasure is behind one of these two doors unfortunately behind the other door is immediate actually not immediate slow and painful death and all of the misery in the world so I want to pick the door carefully and besides his doors there are two people sitting there there's one guy that looks like this and then there's another guy that looks like this and I know for a fact the the gods of brainteasers have come and told me that both of these guys know which door has the treasure and both of these guys know which door has the the unbelievable despair that that would ensue if I open that door so I both of these guys know which door I should be opening and I also know and both of them know that one of them always tells the truth one is always honest always honest and then one of them always lies always one always lies and the problem is I don't know who's who so and I only have one chance to one obviously I can't just open the door and close it if I open the wrong door the despair will come out and kill and and you know all of the pain will happen and I can only ask one question of one of them to decide which door to open so remember they both know the correct answer but one of them always lies one of them always tells the truth and I only have one question to ask of one of them so the brainteaser is is what question do you ask who do you ask it to do you ask it to the green guy or to the brown guy and then what do you do based on what they tell you so that is the the problem description and I will now give you the solution so if you don't want the solution I encourage you not to I think if you think about this one for maybe 20 or 30 minutes or most a day sometimes when you sleep on a brainteaser you it might come to you so I encourage you to do that so pause it stop it whatever don't listen anymore because here is the solution so what you do is I mean you don't know who's who so it's kind of it's not like there's a correct answer on who you ask a question of but what you do is you ask either of the people you ask them what door or what would the other guy say what would the other guy say is the correct door to open correct door to open so let's think about this so I could ask this guy what would the I'm asking the green guy what would this guy say is the correct door to open now what happens if if this guy tells the truth if this guy tells the truth then this guy is the liar right and if and so he will actually tell me what this guy would tell me to do and the liar oh and is the correct right so if I if what would the other guy say is the correct door to open right so if this guy if you were to tell him the correct door to open he would tell you the incorrect door to open right so when you ask this guy what would that guy say he'll be honest about what this guy would say and since he is a liar he would tell you incorrect door so the answer to this question you would get the incorrect door incorrect door would be the answer to that question if this guy is a truth teller and this guy is the liar now what happens if it's the other way around what if I ask this guy and he happens to be the liar so this guy's a liar this guy's the truth teller so the liar will he he can't help himself he will lie so he knows that this other guy is the truth teller there right there's only two of them there he knows this guy's the truth teller and he knows that this guy when asked will say the correct door to open so if you asked if you happen to be lucky in you ask this guy the correct door he would tell you tell it to you this guy will lie and he toes let's say that this is the correct door let's say this is the correct door door door number two is the correct door whoops I somehow zoomed in so let's say the door number two is the correct door if you ask this guy directly you say oh yeah door number two that's you'll find all your happiness is behind door number two now if this guy is a liar he knows that the truth teller will say door number two but he's a liar so he'll tell you door number one so once again if I ask the same question of the liar I still get the incorrect door right so no matter who I asked this question to their answer will be the wrong door right the incorrect door or you know the door that I get all the pain from so what you do is you ask this question what would the other guy say is the correct door to open and no matter who you ask that to they're going to answer the incorrect door and this will you do the opposite so I asked this guy I asked mr. green guy what would mr. brown guy say is the correct door to open and if mr. green guy says Oh open door number green guy will say brown guy will tell you to open door number one well the green guy says that brown guy will tell me to open door number one then I should open up door number two and likewise if green guy says Brown guy we'll say to open door number two then I should open door number one anyway that's a nice warm-up brainteaser for the next one I'm going to give you