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Hey I have a question where i think i am doing everything correct but I don’t get the expected solution, I think the solution may be wrong - or I am....

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Calculate P(0I1) and P(1I1)

for $$P(X=1|Y=1) =P(X=1,Y=1)/P(Y=1)=0.3/0.5=3/5$$

And $$P(X=0|Y=1) = P(X=0,Y=1)/P(Y=1)= 2/5$$

But this is apparently not correct.... maybe you could tell me if i am wrong or not...

Many thanks

Lillys
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  • What is $P(0 | 1)$? You have to specify which random variable gave $0$ and which gave $1$, e.g. $P(X = 0 | Y = 1)$. – snar Jan 03 '19 at 19:07
  • Please use MathJax. https://math.stackexchange.com/help/notation – David K Jan 03 '19 at 19:09
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    It might help if you also wrote your own working in more detail. For example in $0.3/0.5$ there is no indication why you wrote those particular numbers $0.3$ and $0.5.$ It looks like they should be probabilities of something, but you should say what. – David K Jan 03 '19 at 19:11
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    You are plugging wrong numbers in your last calculations ... – Math-fun Jan 03 '19 at 19:15
  • @DavidK I added additional info, many thanks for your help – Lillys Jan 03 '19 at 19:16
  • @Math-fun I edited it, it is now how i believe is correct. – Lillys Jan 03 '19 at 19:18
  • After the edit that swapped the $0$ and $1,$ I agree with your calculations. – David K Jan 03 '19 at 19:20
  • @DavidK many thanks the solution says 0.8 and 0.2.... it is so frustrating if you calculate for the nth time and still can’t find what is wrong – Lillys Jan 03 '19 at 19:24
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    Looks good to me. Obvious check, sum of answers = 1. – herb steinberg Jan 03 '19 at 19:25
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    $0.8$ and $0.2$ would make sense if it were $P(X=0\mid Y=0)=0.8$ and $P(X=1\mid Y=0)=0.2.$ But if it says $Y=1$ then I think your results are the correct ones. It is possible the answer key is wrong--doesn't happen very often, but it does happen sometimes. – David K Jan 03 '19 at 20:00

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