JEE Main & Advanced Sample Paper JEE Main - Mock Test - 2

  • 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.
    \[\therefore \]  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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