EPTOne coin is heads on one side and tails on the other side.
The other coin is heads on both sides.
You draw one coin at random and it is heads.
What is the probability that the other side is heads?
Lack of sanity
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helpman170
34 Posts
One coin is heads on one side and tails on the other side. The other coin is heads on both sides. You draw one coin at random and it is heads. What is the probability that the other side is heads? | ||
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Saechiis
Netherlands4989 Posts
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Ovid
United Kingdom948 Posts
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KelsierSC
United Kingdom10443 Posts
If you select a coin and it has heads then isn't it more likely that the coin you have is the one with 2 heads, therefore it is higher than 50% that the other side is heads? | ||
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KelsierSC
United Kingdom10443 Posts
= 0.5 / 0.75 = 0.667 66% chance | ||
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Saechiis
Netherlands4989 Posts
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KelsierSC
United Kingdom10443 Posts
On May 09 2015 23:17 Saechiis wrote: I don't math. But my thought was that if you get the double heads coin it's always heads on the other side and if you get the split side coin it's always tails on the other side. It's 50% to get either coin. the fact that you pulled a heads on the first coin means you have to use conditional probability. in english words, because the first face is heads you have more than likely pulled the coin with both heads. I think | ||
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Ej_
47656 Posts
H`, H ...... H T H, H` ..... T H H, H` ..... H T T, H ...... H, H` H, T ....... H`, H H, T ....... H, H` exactly half of the events are a solution though? I marked the coin with only heads as having H and H`. Could someone good at maths explain what I'm missing here? e: I guess what Kelsier just said makes sense, then indeed it is 2/3 | ||
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KelsierSC
United Kingdom10443 Posts
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Saechiis
Netherlands4989 Posts
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KelsierSC
United Kingdom10443 Posts
On May 09 2015 23:37 Saechiis wrote: I think you're right KelsierSC. there's always a first time. | ||
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Ovid
United Kingdom948 Posts
On May 09 2015 23:04 KelsierSC wrote: 66% chance That makes sense because of the wording (since you're quantifying that is has to be the double heads coin you pick out not getting a heads on the otherside as an outcome) It's 75% of the time you're going to pick out heads to start and 50% chance for the next outcome so 66% is right just didn't read that the parameter is you've got to have picked out heads to start with. | ||
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LegalLord
United Kingdom13775 Posts
P(A and B) = P(A given B) * P(B) Do the math and you get the result that he got. More information: http://en.wikipedia.org/wiki/Bayes'_rule | ||
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Yorbon
Netherlands4272 Posts
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IAmWithStupid
Russian Federation1016 Posts
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JieXian
Malaysia4677 Posts
I thought it was 75%? If the first coin is already heads, the actual problem changes to: I have 2 coins, one of them has heads on both sides and one of them has heads on one side and tails on the other. I pick one of them at random and flip it. What is the possibility of it coming up heads? so for coin 1 and coin 2 the possibilities are: h1 , h'1 , h2, t2 P(h) = 0.75 | ||
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KelsierSC
United Kingdom10443 Posts
On May 10 2015 03:49 JieXian wrote: Edit: never mind I misunderstood the question, I thought it was about flipping one coin and getting heads, then flipping the other coin. I thought it was 75%? If the first coin is already heads, the actual problem changes to: I have 2 coins, one of them has heads on both sides and one of them has heads on one side and tails on the other. I pick one of them at random and flip it. What is the possibility of it coming up heads? so for coin 1 and coin 2 the possibilities are: h1 , h'1 , h2, t2 P(h) = 0.75 you have neglected the fact that the first face is heads. edit - didn't see edit | ||
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OtherWorld
France17333 Posts
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KelsierSC
United Kingdom10443 Posts
On May 10 2015 04:56 OtherWorld wrote: It's indeed 2/3, but I don't get where the lack of sanity is d: I'm guessing the OP was having issues with the problem. seems like he has left after we did his homework | ||
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riotjune
United States3393 Posts
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