JEE Main & Advanced Mathematics Probability Question Bank Conditional probability bayes theorem

  • question_answer
    A letter is known to have come either from LONDON or CLIFTON; on the postmark only the two consecutive letters ON are legible. The probability that it came from LONDON is

    A)                 \[\frac{5}{17}\]    

    B)                 \[\frac{12}{17}\]

    C)                 \[\frac{17}{30}\]  

    D)                 \[\frac{3}{5}\]

    Correct Answer: B

    Solution :

               We define the following events :            \[{{A}_{1}}:\] Selecting a pair of consecutive letter from the word LONDON.            \[{{A}_{2}}:\] Selecting a pair of consecutive letters from the word CLIFTON.            E : Selecting a pair of letters ?ON?.            Then \[P({{A}_{1}}\cap E)=\frac{2}{5};\] as there are 5 pairs of consecutive letters out of which 2 are ON.            \[P({{A}_{2}}\cap E)=\frac{1}{6};\] as there are 6 pairs of consecutive letters of which one is ON.            \ The required probability is                 \[P\left( \frac{{{A}_{1}}}{E} \right)\]\[=\frac{P({{A}_{1}}\cap E)}{P({{A}_{1}}\cap E)+P({{A}_{2}}\cap E)}=\frac{\frac{2}{5}}{\frac{2}{5}+\frac{1}{6}}=\frac{12}{17}.\]


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