Probability that in one rolling of 5 dice we obtain two '6' and one '5'?

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First I made a mistake of not taking into account that this event is dependent.



So to get two '$6$' the probability would be:



$$ P_5(2) = frac5!2!(5-2)! cdot left(frac16right)^2cdotleft(frac56right)^3 $$



Where



  • $p = dfrac16$

  • $q = dfrac56$


  • $N= 5$ (the number of elements of the system)

Then there are $3$ dice left on the table and we want to know the probability that one of them is a '$5$'. We rule out the two dice with the '$6$' face on. So the number of element in the system is now $3$. So the probability would be:



$$ P_3(1) = frac3!1!(3-1)! cdot left(frac16right)^1cdotleft(frac56right)^2 $$



But then the professor told me that $p=dfrac15$ and not $p=dfrac16$ That's what I don't get.



Of course at the end we multiply those two probabilities to get the final probability that we want.










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  • 2




    The probability is $1/5$ because you CAN"T get another $6.$ Then, you would have more than two $6$s.
    – StatGuy
    3 hours ago















up vote
4
down vote

favorite
1












First I made a mistake of not taking into account that this event is dependent.



So to get two '$6$' the probability would be:



$$ P_5(2) = frac5!2!(5-2)! cdot left(frac16right)^2cdotleft(frac56right)^3 $$



Where



  • $p = dfrac16$

  • $q = dfrac56$


  • $N= 5$ (the number of elements of the system)

Then there are $3$ dice left on the table and we want to know the probability that one of them is a '$5$'. We rule out the two dice with the '$6$' face on. So the number of element in the system is now $3$. So the probability would be:



$$ P_3(1) = frac3!1!(3-1)! cdot left(frac16right)^1cdotleft(frac56right)^2 $$



But then the professor told me that $p=dfrac15$ and not $p=dfrac16$ That's what I don't get.



Of course at the end we multiply those two probabilities to get the final probability that we want.










share|cite|improve this question









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  • 2




    The probability is $1/5$ because you CAN"T get another $6.$ Then, you would have more than two $6$s.
    – StatGuy
    3 hours ago













up vote
4
down vote

favorite
1









up vote
4
down vote

favorite
1






1





First I made a mistake of not taking into account that this event is dependent.



So to get two '$6$' the probability would be:



$$ P_5(2) = frac5!2!(5-2)! cdot left(frac16right)^2cdotleft(frac56right)^3 $$



Where



  • $p = dfrac16$

  • $q = dfrac56$


  • $N= 5$ (the number of elements of the system)

Then there are $3$ dice left on the table and we want to know the probability that one of them is a '$5$'. We rule out the two dice with the '$6$' face on. So the number of element in the system is now $3$. So the probability would be:



$$ P_3(1) = frac3!1!(3-1)! cdot left(frac16right)^1cdotleft(frac56right)^2 $$



But then the professor told me that $p=dfrac15$ and not $p=dfrac16$ That's what I don't get.



Of course at the end we multiply those two probabilities to get the final probability that we want.










share|cite|improve this question









New contributor




Nuz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











First I made a mistake of not taking into account that this event is dependent.



So to get two '$6$' the probability would be:



$$ P_5(2) = frac5!2!(5-2)! cdot left(frac16right)^2cdotleft(frac56right)^3 $$



Where



  • $p = dfrac16$

  • $q = dfrac56$


  • $N= 5$ (the number of elements of the system)

Then there are $3$ dice left on the table and we want to know the probability that one of them is a '$5$'. We rule out the two dice with the '$6$' face on. So the number of element in the system is now $3$. So the probability would be:



$$ P_3(1) = frac3!1!(3-1)! cdot left(frac16right)^1cdotleft(frac56right)^2 $$



But then the professor told me that $p=dfrac15$ and not $p=dfrac16$ That's what I don't get.



Of course at the end we multiply those two probabilities to get the final probability that we want.







probability dice






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edited 39 mins ago









Tianlalu

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  • 2




    The probability is $1/5$ because you CAN"T get another $6.$ Then, you would have more than two $6$s.
    – StatGuy
    3 hours ago













  • 2




    The probability is $1/5$ because you CAN"T get another $6.$ Then, you would have more than two $6$s.
    – StatGuy
    3 hours ago








2




2




The probability is $1/5$ because you CAN"T get another $6.$ Then, you would have more than two $6$s.
– StatGuy
3 hours ago





The probability is $1/5$ because you CAN"T get another $6.$ Then, you would have more than two $6$s.
– StatGuy
3 hours ago











2 Answers
2






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up vote
3
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I am a beginner so see if this makes sense to you. If not let me know. Here are two ways you can solve this.



Method 1



You can solve this using the joint pmf for a multinomial distribution.



$$P(X_6 = 2, X_5 = 1, X_other = 2) = frac5!2!1!2! left( frac16 right)^2 left( frac16 right) left( frac46 right)^2 approx .062$$




Method 2



Alternatively you can solve this using the multiplication rule. Here you get the $1/5$ that your professor talked about because when you condition you are on a reduced sample space.



Let $E$ be the event you get two sixes.



Let $F$ be the event you get one five.



Let $G$ be the event you get two of the others.



$$P(EFG) = P(E)P(F mid E)P(G mid EF) $$



$$P(E) = 5 choose 2left( frac16 right)^2 left( frac56 right)^3 $$



$$P(F mid E) = 3 choose 1left( frac15 right) left( frac45 right)^2$$



$$P(G mid EF) = 2 choose 2left( frac44 right)^2 left( frac44 right)^0 = 1$$



$$P(EFG) approx .062$$






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  • The first method looks like an easy way, can I get the general equation? But still, the $1/5$ doesn't compute to me. How can we rule out a face when it can still happen? To me if we put $p=1/5$ it's would be like considering the 3 other dice have five faces.
    – Nuz
    3 hours ago










  • In regards to method 1 just look up the Multinomial distribution on wiki and check out the pmf. In regards to the $1/5$, personally I don't think about it as a dice with five faces. Personally the way I think about it is you roll the other $3$ dice and if you get any $6$'s you just roll again. So for example, what's the probability you roll a $1$ given that you don't roll a $2,3,4,5,6$? The probability is $1$. Does that mean you have a one faced die? Maybe the way to think of it is if you get something that is not a $1$ you just re roll until you finally do. You are on a reduced sample space.
    – HJ_beginner
    3 hours ago











  • Is there another approach, to calculate the fact that one out of the 3 dice must be a '5' and the 2 others can't be '6's or '5's? Or maybe another analogy or something?
    – Nuz
    1 hour ago










  • Hmmm not sure I understand your question. You can calculate $P(EFG)$ by conditioning on any event first. $P(EFG) = P(G)P(F mid G)P(E mid FG)$. There's many many valid approaches. Part of the reason why I find probability challenging/interesting.
    – HJ_beginner
    1 hour ago

















up vote
2
down vote














But then the professor told me that $p=1/5$ and not $p=1/6$ That's what I don't get.




You have correctly evaluated the probability for the event that two from the five dice show 6.



$$mathsf P(N_6=2)=binom 52 dfrac1^25^36^5$$



Now, of the three dice which don't show 6, each may show faces $1,2,3,4,5$ with equal probability.   Thus the conditional probability for a die showing 5, when given that it does not show 6, is $1/5$.   So the conditional probability for obtaining one 5 when given that two 6 have been obtained among the five dice is:



$$mathsf P(N_5=1mid N_6=2)=binom 31dfrac1^14^25^3$$




Of course at the end we multiply those two probabilities to get the final probability that we want.




And the $5^3$ factors will cancel, so:



$$mathsf P(N_6=2,N_5=1)=binom52binom31dfrac1^21^14^26^5$$



PS: This is called a multinomial distribution.






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    2 Answers
    2






    active

    oldest

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    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    3
    down vote













    I am a beginner so see if this makes sense to you. If not let me know. Here are two ways you can solve this.



    Method 1



    You can solve this using the joint pmf for a multinomial distribution.



    $$P(X_6 = 2, X_5 = 1, X_other = 2) = frac5!2!1!2! left( frac16 right)^2 left( frac16 right) left( frac46 right)^2 approx .062$$




    Method 2



    Alternatively you can solve this using the multiplication rule. Here you get the $1/5$ that your professor talked about because when you condition you are on a reduced sample space.



    Let $E$ be the event you get two sixes.



    Let $F$ be the event you get one five.



    Let $G$ be the event you get two of the others.



    $$P(EFG) = P(E)P(F mid E)P(G mid EF) $$



    $$P(E) = 5 choose 2left( frac16 right)^2 left( frac56 right)^3 $$



    $$P(F mid E) = 3 choose 1left( frac15 right) left( frac45 right)^2$$



    $$P(G mid EF) = 2 choose 2left( frac44 right)^2 left( frac44 right)^0 = 1$$



    $$P(EFG) approx .062$$






    share|cite|improve this answer




















    • The first method looks like an easy way, can I get the general equation? But still, the $1/5$ doesn't compute to me. How can we rule out a face when it can still happen? To me if we put $p=1/5$ it's would be like considering the 3 other dice have five faces.
      – Nuz
      3 hours ago










    • In regards to method 1 just look up the Multinomial distribution on wiki and check out the pmf. In regards to the $1/5$, personally I don't think about it as a dice with five faces. Personally the way I think about it is you roll the other $3$ dice and if you get any $6$'s you just roll again. So for example, what's the probability you roll a $1$ given that you don't roll a $2,3,4,5,6$? The probability is $1$. Does that mean you have a one faced die? Maybe the way to think of it is if you get something that is not a $1$ you just re roll until you finally do. You are on a reduced sample space.
      – HJ_beginner
      3 hours ago











    • Is there another approach, to calculate the fact that one out of the 3 dice must be a '5' and the 2 others can't be '6's or '5's? Or maybe another analogy or something?
      – Nuz
      1 hour ago










    • Hmmm not sure I understand your question. You can calculate $P(EFG)$ by conditioning on any event first. $P(EFG) = P(G)P(F mid G)P(E mid FG)$. There's many many valid approaches. Part of the reason why I find probability challenging/interesting.
      – HJ_beginner
      1 hour ago














    up vote
    3
    down vote













    I am a beginner so see if this makes sense to you. If not let me know. Here are two ways you can solve this.



    Method 1



    You can solve this using the joint pmf for a multinomial distribution.



    $$P(X_6 = 2, X_5 = 1, X_other = 2) = frac5!2!1!2! left( frac16 right)^2 left( frac16 right) left( frac46 right)^2 approx .062$$




    Method 2



    Alternatively you can solve this using the multiplication rule. Here you get the $1/5$ that your professor talked about because when you condition you are on a reduced sample space.



    Let $E$ be the event you get two sixes.



    Let $F$ be the event you get one five.



    Let $G$ be the event you get two of the others.



    $$P(EFG) = P(E)P(F mid E)P(G mid EF) $$



    $$P(E) = 5 choose 2left( frac16 right)^2 left( frac56 right)^3 $$



    $$P(F mid E) = 3 choose 1left( frac15 right) left( frac45 right)^2$$



    $$P(G mid EF) = 2 choose 2left( frac44 right)^2 left( frac44 right)^0 = 1$$



    $$P(EFG) approx .062$$






    share|cite|improve this answer




















    • The first method looks like an easy way, can I get the general equation? But still, the $1/5$ doesn't compute to me. How can we rule out a face when it can still happen? To me if we put $p=1/5$ it's would be like considering the 3 other dice have five faces.
      – Nuz
      3 hours ago










    • In regards to method 1 just look up the Multinomial distribution on wiki and check out the pmf. In regards to the $1/5$, personally I don't think about it as a dice with five faces. Personally the way I think about it is you roll the other $3$ dice and if you get any $6$'s you just roll again. So for example, what's the probability you roll a $1$ given that you don't roll a $2,3,4,5,6$? The probability is $1$. Does that mean you have a one faced die? Maybe the way to think of it is if you get something that is not a $1$ you just re roll until you finally do. You are on a reduced sample space.
      – HJ_beginner
      3 hours ago











    • Is there another approach, to calculate the fact that one out of the 3 dice must be a '5' and the 2 others can't be '6's or '5's? Or maybe another analogy or something?
      – Nuz
      1 hour ago










    • Hmmm not sure I understand your question. You can calculate $P(EFG)$ by conditioning on any event first. $P(EFG) = P(G)P(F mid G)P(E mid FG)$. There's many many valid approaches. Part of the reason why I find probability challenging/interesting.
      – HJ_beginner
      1 hour ago












    up vote
    3
    down vote










    up vote
    3
    down vote









    I am a beginner so see if this makes sense to you. If not let me know. Here are two ways you can solve this.



    Method 1



    You can solve this using the joint pmf for a multinomial distribution.



    $$P(X_6 = 2, X_5 = 1, X_other = 2) = frac5!2!1!2! left( frac16 right)^2 left( frac16 right) left( frac46 right)^2 approx .062$$




    Method 2



    Alternatively you can solve this using the multiplication rule. Here you get the $1/5$ that your professor talked about because when you condition you are on a reduced sample space.



    Let $E$ be the event you get two sixes.



    Let $F$ be the event you get one five.



    Let $G$ be the event you get two of the others.



    $$P(EFG) = P(E)P(F mid E)P(G mid EF) $$



    $$P(E) = 5 choose 2left( frac16 right)^2 left( frac56 right)^3 $$



    $$P(F mid E) = 3 choose 1left( frac15 right) left( frac45 right)^2$$



    $$P(G mid EF) = 2 choose 2left( frac44 right)^2 left( frac44 right)^0 = 1$$



    $$P(EFG) approx .062$$






    share|cite|improve this answer












    I am a beginner so see if this makes sense to you. If not let me know. Here are two ways you can solve this.



    Method 1



    You can solve this using the joint pmf for a multinomial distribution.



    $$P(X_6 = 2, X_5 = 1, X_other = 2) = frac5!2!1!2! left( frac16 right)^2 left( frac16 right) left( frac46 right)^2 approx .062$$




    Method 2



    Alternatively you can solve this using the multiplication rule. Here you get the $1/5$ that your professor talked about because when you condition you are on a reduced sample space.



    Let $E$ be the event you get two sixes.



    Let $F$ be the event you get one five.



    Let $G$ be the event you get two of the others.



    $$P(EFG) = P(E)P(F mid E)P(G mid EF) $$



    $$P(E) = 5 choose 2left( frac16 right)^2 left( frac56 right)^3 $$



    $$P(F mid E) = 3 choose 1left( frac15 right) left( frac45 right)^2$$



    $$P(G mid EF) = 2 choose 2left( frac44 right)^2 left( frac44 right)^0 = 1$$



    $$P(EFG) approx .062$$







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered 3 hours ago









    HJ_beginner

    787215




    787215











    • The first method looks like an easy way, can I get the general equation? But still, the $1/5$ doesn't compute to me. How can we rule out a face when it can still happen? To me if we put $p=1/5$ it's would be like considering the 3 other dice have five faces.
      – Nuz
      3 hours ago










    • In regards to method 1 just look up the Multinomial distribution on wiki and check out the pmf. In regards to the $1/5$, personally I don't think about it as a dice with five faces. Personally the way I think about it is you roll the other $3$ dice and if you get any $6$'s you just roll again. So for example, what's the probability you roll a $1$ given that you don't roll a $2,3,4,5,6$? The probability is $1$. Does that mean you have a one faced die? Maybe the way to think of it is if you get something that is not a $1$ you just re roll until you finally do. You are on a reduced sample space.
      – HJ_beginner
      3 hours ago











    • Is there another approach, to calculate the fact that one out of the 3 dice must be a '5' and the 2 others can't be '6's or '5's? Or maybe another analogy or something?
      – Nuz
      1 hour ago










    • Hmmm not sure I understand your question. You can calculate $P(EFG)$ by conditioning on any event first. $P(EFG) = P(G)P(F mid G)P(E mid FG)$. There's many many valid approaches. Part of the reason why I find probability challenging/interesting.
      – HJ_beginner
      1 hour ago
















    • The first method looks like an easy way, can I get the general equation? But still, the $1/5$ doesn't compute to me. How can we rule out a face when it can still happen? To me if we put $p=1/5$ it's would be like considering the 3 other dice have five faces.
      – Nuz
      3 hours ago










    • In regards to method 1 just look up the Multinomial distribution on wiki and check out the pmf. In regards to the $1/5$, personally I don't think about it as a dice with five faces. Personally the way I think about it is you roll the other $3$ dice and if you get any $6$'s you just roll again. So for example, what's the probability you roll a $1$ given that you don't roll a $2,3,4,5,6$? The probability is $1$. Does that mean you have a one faced die? Maybe the way to think of it is if you get something that is not a $1$ you just re roll until you finally do. You are on a reduced sample space.
      – HJ_beginner
      3 hours ago











    • Is there another approach, to calculate the fact that one out of the 3 dice must be a '5' and the 2 others can't be '6's or '5's? Or maybe another analogy or something?
      – Nuz
      1 hour ago










    • Hmmm not sure I understand your question. You can calculate $P(EFG)$ by conditioning on any event first. $P(EFG) = P(G)P(F mid G)P(E mid FG)$. There's many many valid approaches. Part of the reason why I find probability challenging/interesting.
      – HJ_beginner
      1 hour ago















    The first method looks like an easy way, can I get the general equation? But still, the $1/5$ doesn't compute to me. How can we rule out a face when it can still happen? To me if we put $p=1/5$ it's would be like considering the 3 other dice have five faces.
    – Nuz
    3 hours ago




    The first method looks like an easy way, can I get the general equation? But still, the $1/5$ doesn't compute to me. How can we rule out a face when it can still happen? To me if we put $p=1/5$ it's would be like considering the 3 other dice have five faces.
    – Nuz
    3 hours ago












    In regards to method 1 just look up the Multinomial distribution on wiki and check out the pmf. In regards to the $1/5$, personally I don't think about it as a dice with five faces. Personally the way I think about it is you roll the other $3$ dice and if you get any $6$'s you just roll again. So for example, what's the probability you roll a $1$ given that you don't roll a $2,3,4,5,6$? The probability is $1$. Does that mean you have a one faced die? Maybe the way to think of it is if you get something that is not a $1$ you just re roll until you finally do. You are on a reduced sample space.
    – HJ_beginner
    3 hours ago





    In regards to method 1 just look up the Multinomial distribution on wiki and check out the pmf. In regards to the $1/5$, personally I don't think about it as a dice with five faces. Personally the way I think about it is you roll the other $3$ dice and if you get any $6$'s you just roll again. So for example, what's the probability you roll a $1$ given that you don't roll a $2,3,4,5,6$? The probability is $1$. Does that mean you have a one faced die? Maybe the way to think of it is if you get something that is not a $1$ you just re roll until you finally do. You are on a reduced sample space.
    – HJ_beginner
    3 hours ago













    Is there another approach, to calculate the fact that one out of the 3 dice must be a '5' and the 2 others can't be '6's or '5's? Or maybe another analogy or something?
    – Nuz
    1 hour ago




    Is there another approach, to calculate the fact that one out of the 3 dice must be a '5' and the 2 others can't be '6's or '5's? Or maybe another analogy or something?
    – Nuz
    1 hour ago












    Hmmm not sure I understand your question. You can calculate $P(EFG)$ by conditioning on any event first. $P(EFG) = P(G)P(F mid G)P(E mid FG)$. There's many many valid approaches. Part of the reason why I find probability challenging/interesting.
    – HJ_beginner
    1 hour ago




    Hmmm not sure I understand your question. You can calculate $P(EFG)$ by conditioning on any event first. $P(EFG) = P(G)P(F mid G)P(E mid FG)$. There's many many valid approaches. Part of the reason why I find probability challenging/interesting.
    – HJ_beginner
    1 hour ago










    up vote
    2
    down vote














    But then the professor told me that $p=1/5$ and not $p=1/6$ That's what I don't get.




    You have correctly evaluated the probability for the event that two from the five dice show 6.



    $$mathsf P(N_6=2)=binom 52 dfrac1^25^36^5$$



    Now, of the three dice which don't show 6, each may show faces $1,2,3,4,5$ with equal probability.   Thus the conditional probability for a die showing 5, when given that it does not show 6, is $1/5$.   So the conditional probability for obtaining one 5 when given that two 6 have been obtained among the five dice is:



    $$mathsf P(N_5=1mid N_6=2)=binom 31dfrac1^14^25^3$$




    Of course at the end we multiply those two probabilities to get the final probability that we want.




    And the $5^3$ factors will cancel, so:



    $$mathsf P(N_6=2,N_5=1)=binom52binom31dfrac1^21^14^26^5$$



    PS: This is called a multinomial distribution.






    share|cite|improve this answer
























      up vote
      2
      down vote














      But then the professor told me that $p=1/5$ and not $p=1/6$ That's what I don't get.




      You have correctly evaluated the probability for the event that two from the five dice show 6.



      $$mathsf P(N_6=2)=binom 52 dfrac1^25^36^5$$



      Now, of the three dice which don't show 6, each may show faces $1,2,3,4,5$ with equal probability.   Thus the conditional probability for a die showing 5, when given that it does not show 6, is $1/5$.   So the conditional probability for obtaining one 5 when given that two 6 have been obtained among the five dice is:



      $$mathsf P(N_5=1mid N_6=2)=binom 31dfrac1^14^25^3$$




      Of course at the end we multiply those two probabilities to get the final probability that we want.




      And the $5^3$ factors will cancel, so:



      $$mathsf P(N_6=2,N_5=1)=binom52binom31dfrac1^21^14^26^5$$



      PS: This is called a multinomial distribution.






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        But then the professor told me that $p=1/5$ and not $p=1/6$ That's what I don't get.




        You have correctly evaluated the probability for the event that two from the five dice show 6.



        $$mathsf P(N_6=2)=binom 52 dfrac1^25^36^5$$



        Now, of the three dice which don't show 6, each may show faces $1,2,3,4,5$ with equal probability.   Thus the conditional probability for a die showing 5, when given that it does not show 6, is $1/5$.   So the conditional probability for obtaining one 5 when given that two 6 have been obtained among the five dice is:



        $$mathsf P(N_5=1mid N_6=2)=binom 31dfrac1^14^25^3$$




        Of course at the end we multiply those two probabilities to get the final probability that we want.




        And the $5^3$ factors will cancel, so:



        $$mathsf P(N_6=2,N_5=1)=binom52binom31dfrac1^21^14^26^5$$



        PS: This is called a multinomial distribution.






        share|cite|improve this answer













        But then the professor told me that $p=1/5$ and not $p=1/6$ That's what I don't get.




        You have correctly evaluated the probability for the event that two from the five dice show 6.



        $$mathsf P(N_6=2)=binom 52 dfrac1^25^36^5$$



        Now, of the three dice which don't show 6, each may show faces $1,2,3,4,5$ with equal probability.   Thus the conditional probability for a die showing 5, when given that it does not show 6, is $1/5$.   So the conditional probability for obtaining one 5 when given that two 6 have been obtained among the five dice is:



        $$mathsf P(N_5=1mid N_6=2)=binom 31dfrac1^14^25^3$$




        Of course at the end we multiply those two probabilities to get the final probability that we want.




        And the $5^3$ factors will cancel, so:



        $$mathsf P(N_6=2,N_5=1)=binom52binom31dfrac1^21^14^26^5$$



        PS: This is called a multinomial distribution.







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        share|cite|improve this answer



        share|cite|improve this answer










        answered 22 mins ago









        Graham Kemp

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        82.2k43377




















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