Two coins, one experiment: what is the probability that the coin lands on heads on exactly $7$ of the $10$...












0












$begingroup$


When coin $1$ is flipped, it lands on heads with probability $0.4$; when coin $2$ is flipped, it lands on heads with probability $0.7$. One of these coins is randomly chosen and flipped $10$ times.



(a) What is the probability that the coin lands on heads on
exactly $7$ of the $10$ flips?



(b) Given that the first of these $10$ flips lands heads, what is the conditional probability that exactly $7$ of the $10$ flips land on heads?



MY ATTEMPT



(a) Let us denote by $X$ the random variable which counts the numbers of heads in 10 flips. Thus $Xsimtext{Binomial}(10,p)$, where the parameter $p$ depends on the coin that is chosen: $C = 1$ stands for the event "the first coin is chosen" and $C = 2$ stands for the event "the second coin is chosen". Based on these considerations, we have the answer to the first question, which is



begin{align*}
textbf{P}(X = 7) = textbf{P}(X = 7mid C = 1)textbf{P}(C = 1) + textbf{P}(X = 7mid C = 2)textbf{P}(C = 2)
end{align*}



where
begin{cases}
textbf{P}(X = 7mid C = 1) = displaystyle{10choose 7}(0.4)^{7}(0,6)^{3}\\
textbf{P}(X = 7mid C = 2) = displaystyle{10choose 7}(0.7)^{7}(0,3)^{3}\\
textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
end{cases}



(b) Since $X = displaystylesum_{k=1}^{10} X_{k}$, where each $X_{k}simtext{Bernoulli}(p)$, we are interested in the event $textbf{P}(X = 7mid X_{1} = 1)$, which is equivalent to



begin{align*}
textbf{P}(X = 7mid X_{1} = 1) & = frac{textbf{P}(X = 7, X_{1} = 1)}{textbf{P}(X_{1} = 1)} = frac{textbf{P}left(displaystylesum_{k=2}^{10}X_{k} = 6right)}{textbf{P}(X_{1} = 1)}
end{align*}



Since $textbf{P}(X_{1} = 1) = textbf{P}(X_{1} = 1 | C = 1)textbf{P}(C = 1) + textbf{P}(X_{1} = 1 | C = 2)textbf{P}(C = 2)$, it suffices to calculate $textbf{P}(Y = 6)$, where $Y := displaystylesum_{k=2}^{10}X_{k}$ and, consequentely, $Ysimtext{Binomial}(9,p)$:



begin{align*}
textbf{P}(Y = 6) = textbf{P}(Y = 6mid C = 1)textbf{P}(C = 1) + textbf{P}(Y = 6mid C = 2)textbf{P}(C = 2)
end{align*}



where
begin{cases}
textbf{P}(Y = 6mid C = 1) = displaystyle{9choose 6}(0.4)^{6}(0,6)^{3}\\
textbf{P}(Y = 6mid C = 2) = displaystyle{9choose 6}(0.7)^{6}(0,3)^{3}\\
textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
end{cases}



My question is: am I working it right or is there any conceptual misapplication?



Any contribution is appreciated. Thanks in advance!










share|cite|improve this question











$endgroup$

















    0












    $begingroup$


    When coin $1$ is flipped, it lands on heads with probability $0.4$; when coin $2$ is flipped, it lands on heads with probability $0.7$. One of these coins is randomly chosen and flipped $10$ times.



    (a) What is the probability that the coin lands on heads on
    exactly $7$ of the $10$ flips?



    (b) Given that the first of these $10$ flips lands heads, what is the conditional probability that exactly $7$ of the $10$ flips land on heads?



    MY ATTEMPT



    (a) Let us denote by $X$ the random variable which counts the numbers of heads in 10 flips. Thus $Xsimtext{Binomial}(10,p)$, where the parameter $p$ depends on the coin that is chosen: $C = 1$ stands for the event "the first coin is chosen" and $C = 2$ stands for the event "the second coin is chosen". Based on these considerations, we have the answer to the first question, which is



    begin{align*}
    textbf{P}(X = 7) = textbf{P}(X = 7mid C = 1)textbf{P}(C = 1) + textbf{P}(X = 7mid C = 2)textbf{P}(C = 2)
    end{align*}



    where
    begin{cases}
    textbf{P}(X = 7mid C = 1) = displaystyle{10choose 7}(0.4)^{7}(0,6)^{3}\\
    textbf{P}(X = 7mid C = 2) = displaystyle{10choose 7}(0.7)^{7}(0,3)^{3}\\
    textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
    end{cases}



    (b) Since $X = displaystylesum_{k=1}^{10} X_{k}$, where each $X_{k}simtext{Bernoulli}(p)$, we are interested in the event $textbf{P}(X = 7mid X_{1} = 1)$, which is equivalent to



    begin{align*}
    textbf{P}(X = 7mid X_{1} = 1) & = frac{textbf{P}(X = 7, X_{1} = 1)}{textbf{P}(X_{1} = 1)} = frac{textbf{P}left(displaystylesum_{k=2}^{10}X_{k} = 6right)}{textbf{P}(X_{1} = 1)}
    end{align*}



    Since $textbf{P}(X_{1} = 1) = textbf{P}(X_{1} = 1 | C = 1)textbf{P}(C = 1) + textbf{P}(X_{1} = 1 | C = 2)textbf{P}(C = 2)$, it suffices to calculate $textbf{P}(Y = 6)$, where $Y := displaystylesum_{k=2}^{10}X_{k}$ and, consequentely, $Ysimtext{Binomial}(9,p)$:



    begin{align*}
    textbf{P}(Y = 6) = textbf{P}(Y = 6mid C = 1)textbf{P}(C = 1) + textbf{P}(Y = 6mid C = 2)textbf{P}(C = 2)
    end{align*}



    where
    begin{cases}
    textbf{P}(Y = 6mid C = 1) = displaystyle{9choose 6}(0.4)^{6}(0,6)^{3}\\
    textbf{P}(Y = 6mid C = 2) = displaystyle{9choose 6}(0.7)^{6}(0,3)^{3}\\
    textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
    end{cases}



    My question is: am I working it right or is there any conceptual misapplication?



    Any contribution is appreciated. Thanks in advance!










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      When coin $1$ is flipped, it lands on heads with probability $0.4$; when coin $2$ is flipped, it lands on heads with probability $0.7$. One of these coins is randomly chosen and flipped $10$ times.



      (a) What is the probability that the coin lands on heads on
      exactly $7$ of the $10$ flips?



      (b) Given that the first of these $10$ flips lands heads, what is the conditional probability that exactly $7$ of the $10$ flips land on heads?



      MY ATTEMPT



      (a) Let us denote by $X$ the random variable which counts the numbers of heads in 10 flips. Thus $Xsimtext{Binomial}(10,p)$, where the parameter $p$ depends on the coin that is chosen: $C = 1$ stands for the event "the first coin is chosen" and $C = 2$ stands for the event "the second coin is chosen". Based on these considerations, we have the answer to the first question, which is



      begin{align*}
      textbf{P}(X = 7) = textbf{P}(X = 7mid C = 1)textbf{P}(C = 1) + textbf{P}(X = 7mid C = 2)textbf{P}(C = 2)
      end{align*}



      where
      begin{cases}
      textbf{P}(X = 7mid C = 1) = displaystyle{10choose 7}(0.4)^{7}(0,6)^{3}\\
      textbf{P}(X = 7mid C = 2) = displaystyle{10choose 7}(0.7)^{7}(0,3)^{3}\\
      textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
      end{cases}



      (b) Since $X = displaystylesum_{k=1}^{10} X_{k}$, where each $X_{k}simtext{Bernoulli}(p)$, we are interested in the event $textbf{P}(X = 7mid X_{1} = 1)$, which is equivalent to



      begin{align*}
      textbf{P}(X = 7mid X_{1} = 1) & = frac{textbf{P}(X = 7, X_{1} = 1)}{textbf{P}(X_{1} = 1)} = frac{textbf{P}left(displaystylesum_{k=2}^{10}X_{k} = 6right)}{textbf{P}(X_{1} = 1)}
      end{align*}



      Since $textbf{P}(X_{1} = 1) = textbf{P}(X_{1} = 1 | C = 1)textbf{P}(C = 1) + textbf{P}(X_{1} = 1 | C = 2)textbf{P}(C = 2)$, it suffices to calculate $textbf{P}(Y = 6)$, where $Y := displaystylesum_{k=2}^{10}X_{k}$ and, consequentely, $Ysimtext{Binomial}(9,p)$:



      begin{align*}
      textbf{P}(Y = 6) = textbf{P}(Y = 6mid C = 1)textbf{P}(C = 1) + textbf{P}(Y = 6mid C = 2)textbf{P}(C = 2)
      end{align*}



      where
      begin{cases}
      textbf{P}(Y = 6mid C = 1) = displaystyle{9choose 6}(0.4)^{6}(0,6)^{3}\\
      textbf{P}(Y = 6mid C = 2) = displaystyle{9choose 6}(0.7)^{6}(0,3)^{3}\\
      textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
      end{cases}



      My question is: am I working it right or is there any conceptual misapplication?



      Any contribution is appreciated. Thanks in advance!










      share|cite|improve this question











      $endgroup$




      When coin $1$ is flipped, it lands on heads with probability $0.4$; when coin $2$ is flipped, it lands on heads with probability $0.7$. One of these coins is randomly chosen and flipped $10$ times.



      (a) What is the probability that the coin lands on heads on
      exactly $7$ of the $10$ flips?



      (b) Given that the first of these $10$ flips lands heads, what is the conditional probability that exactly $7$ of the $10$ flips land on heads?



      MY ATTEMPT



      (a) Let us denote by $X$ the random variable which counts the numbers of heads in 10 flips. Thus $Xsimtext{Binomial}(10,p)$, where the parameter $p$ depends on the coin that is chosen: $C = 1$ stands for the event "the first coin is chosen" and $C = 2$ stands for the event "the second coin is chosen". Based on these considerations, we have the answer to the first question, which is



      begin{align*}
      textbf{P}(X = 7) = textbf{P}(X = 7mid C = 1)textbf{P}(C = 1) + textbf{P}(X = 7mid C = 2)textbf{P}(C = 2)
      end{align*}



      where
      begin{cases}
      textbf{P}(X = 7mid C = 1) = displaystyle{10choose 7}(0.4)^{7}(0,6)^{3}\\
      textbf{P}(X = 7mid C = 2) = displaystyle{10choose 7}(0.7)^{7}(0,3)^{3}\\
      textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
      end{cases}



      (b) Since $X = displaystylesum_{k=1}^{10} X_{k}$, where each $X_{k}simtext{Bernoulli}(p)$, we are interested in the event $textbf{P}(X = 7mid X_{1} = 1)$, which is equivalent to



      begin{align*}
      textbf{P}(X = 7mid X_{1} = 1) & = frac{textbf{P}(X = 7, X_{1} = 1)}{textbf{P}(X_{1} = 1)} = frac{textbf{P}left(displaystylesum_{k=2}^{10}X_{k} = 6right)}{textbf{P}(X_{1} = 1)}
      end{align*}



      Since $textbf{P}(X_{1} = 1) = textbf{P}(X_{1} = 1 | C = 1)textbf{P}(C = 1) + textbf{P}(X_{1} = 1 | C = 2)textbf{P}(C = 2)$, it suffices to calculate $textbf{P}(Y = 6)$, where $Y := displaystylesum_{k=2}^{10}X_{k}$ and, consequentely, $Ysimtext{Binomial}(9,p)$:



      begin{align*}
      textbf{P}(Y = 6) = textbf{P}(Y = 6mid C = 1)textbf{P}(C = 1) + textbf{P}(Y = 6mid C = 2)textbf{P}(C = 2)
      end{align*}



      where
      begin{cases}
      textbf{P}(Y = 6mid C = 1) = displaystyle{9choose 6}(0.4)^{6}(0,6)^{3}\\
      textbf{P}(Y = 6mid C = 2) = displaystyle{9choose 6}(0.7)^{6}(0,3)^{3}\\
      textbf{P}(C = 1) = textbf{P}(C = 2) = 0.5
      end{cases}



      My question is: am I working it right or is there any conceptual misapplication?



      Any contribution is appreciated. Thanks in advance!







      probability probability-theory conditional-probability binomial-distribution






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      edited Jan 27 at 2:45









      Key Flex

      8,63561233




      8,63561233










      asked Jan 27 at 2:21









      user1337user1337

      46310




      46310






















          1 Answer
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          2












          $begingroup$

          For part $a)$,



          Let $K_1$ denotes when coin $1$ is chosen and $K_2$ denotes when coin $2$ is chosen.



          The probability that the coin lands on heads on exactly $7$ of the $10$ flips is $P(H)$
          $$P(H)=P(H|K_1)P(K_1)+P(H|K_2)P(K_2)$$
          $$P(H)=dbinom{10}{7}(0.4)^7(0.6)^3(0.5)+dbinom{10}{7}(0.7)^7(0.3)^3()0.5$$



          For part $b)$,



          When the first flip is head, then the probability is computed from choosing $6$ head flips out of $9$ flips in both coins case. So, we get $dbinom{9}{6}(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$



          Let $T$ denotes the event that the first flip is head.



          $$P(H|T)=dfrac{P(Hcap T)}{P(T)}$$
          $P(T)=P(T|K_1)P(K_1)+P(T|K_2)P(K_2)=(0.4)(0.5)+(0.7)(0.5)=0.55$



          $P(Hcap T)=P(Hcap T|K_1)P(K_1)+P(Hcap T|K_2)P(K_2)$
          $$=dbinom96(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            In the first place, thanks for the contribution Flex! As far as I have checked, we have given the same answers. Maybe there is just one typo: it should be $(0,4)^{6}(0.6)^{3}(0.5)$ and $(0.7)^{6}(0.3)^{3}(0.5)$ in the last line.
            $endgroup$
            – user1337
            Jan 27 at 2:39






          • 1




            $begingroup$
            @user1337 Fixed it! Thanks
            $endgroup$
            – Key Flex
            Jan 27 at 2:44











          Your Answer





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          1 Answer
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          1 Answer
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          active

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          active

          oldest

          votes






          active

          oldest

          votes









          2












          $begingroup$

          For part $a)$,



          Let $K_1$ denotes when coin $1$ is chosen and $K_2$ denotes when coin $2$ is chosen.



          The probability that the coin lands on heads on exactly $7$ of the $10$ flips is $P(H)$
          $$P(H)=P(H|K_1)P(K_1)+P(H|K_2)P(K_2)$$
          $$P(H)=dbinom{10}{7}(0.4)^7(0.6)^3(0.5)+dbinom{10}{7}(0.7)^7(0.3)^3()0.5$$



          For part $b)$,



          When the first flip is head, then the probability is computed from choosing $6$ head flips out of $9$ flips in both coins case. So, we get $dbinom{9}{6}(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$



          Let $T$ denotes the event that the first flip is head.



          $$P(H|T)=dfrac{P(Hcap T)}{P(T)}$$
          $P(T)=P(T|K_1)P(K_1)+P(T|K_2)P(K_2)=(0.4)(0.5)+(0.7)(0.5)=0.55$



          $P(Hcap T)=P(Hcap T|K_1)P(K_1)+P(Hcap T|K_2)P(K_2)$
          $$=dbinom96(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            In the first place, thanks for the contribution Flex! As far as I have checked, we have given the same answers. Maybe there is just one typo: it should be $(0,4)^{6}(0.6)^{3}(0.5)$ and $(0.7)^{6}(0.3)^{3}(0.5)$ in the last line.
            $endgroup$
            – user1337
            Jan 27 at 2:39






          • 1




            $begingroup$
            @user1337 Fixed it! Thanks
            $endgroup$
            – Key Flex
            Jan 27 at 2:44
















          2












          $begingroup$

          For part $a)$,



          Let $K_1$ denotes when coin $1$ is chosen and $K_2$ denotes when coin $2$ is chosen.



          The probability that the coin lands on heads on exactly $7$ of the $10$ flips is $P(H)$
          $$P(H)=P(H|K_1)P(K_1)+P(H|K_2)P(K_2)$$
          $$P(H)=dbinom{10}{7}(0.4)^7(0.6)^3(0.5)+dbinom{10}{7}(0.7)^7(0.3)^3()0.5$$



          For part $b)$,



          When the first flip is head, then the probability is computed from choosing $6$ head flips out of $9$ flips in both coins case. So, we get $dbinom{9}{6}(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$



          Let $T$ denotes the event that the first flip is head.



          $$P(H|T)=dfrac{P(Hcap T)}{P(T)}$$
          $P(T)=P(T|K_1)P(K_1)+P(T|K_2)P(K_2)=(0.4)(0.5)+(0.7)(0.5)=0.55$



          $P(Hcap T)=P(Hcap T|K_1)P(K_1)+P(Hcap T|K_2)P(K_2)$
          $$=dbinom96(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            In the first place, thanks for the contribution Flex! As far as I have checked, we have given the same answers. Maybe there is just one typo: it should be $(0,4)^{6}(0.6)^{3}(0.5)$ and $(0.7)^{6}(0.3)^{3}(0.5)$ in the last line.
            $endgroup$
            – user1337
            Jan 27 at 2:39






          • 1




            $begingroup$
            @user1337 Fixed it! Thanks
            $endgroup$
            – Key Flex
            Jan 27 at 2:44














          2












          2








          2





          $begingroup$

          For part $a)$,



          Let $K_1$ denotes when coin $1$ is chosen and $K_2$ denotes when coin $2$ is chosen.



          The probability that the coin lands on heads on exactly $7$ of the $10$ flips is $P(H)$
          $$P(H)=P(H|K_1)P(K_1)+P(H|K_2)P(K_2)$$
          $$P(H)=dbinom{10}{7}(0.4)^7(0.6)^3(0.5)+dbinom{10}{7}(0.7)^7(0.3)^3()0.5$$



          For part $b)$,



          When the first flip is head, then the probability is computed from choosing $6$ head flips out of $9$ flips in both coins case. So, we get $dbinom{9}{6}(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$



          Let $T$ denotes the event that the first flip is head.



          $$P(H|T)=dfrac{P(Hcap T)}{P(T)}$$
          $P(T)=P(T|K_1)P(K_1)+P(T|K_2)P(K_2)=(0.4)(0.5)+(0.7)(0.5)=0.55$



          $P(Hcap T)=P(Hcap T|K_1)P(K_1)+P(Hcap T|K_2)P(K_2)$
          $$=dbinom96(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$$






          share|cite|improve this answer











          $endgroup$



          For part $a)$,



          Let $K_1$ denotes when coin $1$ is chosen and $K_2$ denotes when coin $2$ is chosen.



          The probability that the coin lands on heads on exactly $7$ of the $10$ flips is $P(H)$
          $$P(H)=P(H|K_1)P(K_1)+P(H|K_2)P(K_2)$$
          $$P(H)=dbinom{10}{7}(0.4)^7(0.6)^3(0.5)+dbinom{10}{7}(0.7)^7(0.3)^3()0.5$$



          For part $b)$,



          When the first flip is head, then the probability is computed from choosing $6$ head flips out of $9$ flips in both coins case. So, we get $dbinom{9}{6}(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$



          Let $T$ denotes the event that the first flip is head.



          $$P(H|T)=dfrac{P(Hcap T)}{P(T)}$$
          $P(T)=P(T|K_1)P(K_1)+P(T|K_2)P(K_2)=(0.4)(0.5)+(0.7)(0.5)=0.55$



          $P(Hcap T)=P(Hcap T|K_1)P(K_1)+P(Hcap T|K_2)P(K_2)$
          $$=dbinom96(0.4)^6(0.6)^3(0.5)+dbinom96(0.7)^6(0.3)^3(0.5)$$







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Jan 27 at 2:43

























          answered Jan 27 at 2:32









          Key FlexKey Flex

          8,63561233




          8,63561233












          • $begingroup$
            In the first place, thanks for the contribution Flex! As far as I have checked, we have given the same answers. Maybe there is just one typo: it should be $(0,4)^{6}(0.6)^{3}(0.5)$ and $(0.7)^{6}(0.3)^{3}(0.5)$ in the last line.
            $endgroup$
            – user1337
            Jan 27 at 2:39






          • 1




            $begingroup$
            @user1337 Fixed it! Thanks
            $endgroup$
            – Key Flex
            Jan 27 at 2:44


















          • $begingroup$
            In the first place, thanks for the contribution Flex! As far as I have checked, we have given the same answers. Maybe there is just one typo: it should be $(0,4)^{6}(0.6)^{3}(0.5)$ and $(0.7)^{6}(0.3)^{3}(0.5)$ in the last line.
            $endgroup$
            – user1337
            Jan 27 at 2:39






          • 1




            $begingroup$
            @user1337 Fixed it! Thanks
            $endgroup$
            – Key Flex
            Jan 27 at 2:44
















          $begingroup$
          In the first place, thanks for the contribution Flex! As far as I have checked, we have given the same answers. Maybe there is just one typo: it should be $(0,4)^{6}(0.6)^{3}(0.5)$ and $(0.7)^{6}(0.3)^{3}(0.5)$ in the last line.
          $endgroup$
          – user1337
          Jan 27 at 2:39




          $begingroup$
          In the first place, thanks for the contribution Flex! As far as I have checked, we have given the same answers. Maybe there is just one typo: it should be $(0,4)^{6}(0.6)^{3}(0.5)$ and $(0.7)^{6}(0.3)^{3}(0.5)$ in the last line.
          $endgroup$
          – user1337
          Jan 27 at 2:39




          1




          1




          $begingroup$
          @user1337 Fixed it! Thanks
          $endgroup$
          – Key Flex
          Jan 27 at 2:44




          $begingroup$
          @user1337 Fixed it! Thanks
          $endgroup$
          – Key Flex
          Jan 27 at 2:44


















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