JEE Main & Advanced Mathematics Probability Question Bank Definition of probability

  • question_answer
    An unbiased die is tossed until a number greater than 4 appears. The probability that an even number of tosses is needed is                                            [IIT 1994]

    A)                 \[\frac{1}{2}\]       

    B)                 \[\frac{2}{5}\]

    C)                 \[\frac{1}{5}\]       

    D)                 \[\frac{2}{3}\]

    Correct Answer: B

    Solution :

               Probability of success \[=\frac{2}{6}=\frac{1}{3}=p\]            Probability of failure \[=1-\frac{1}{3}=\frac{2}{3}=q\]            Probability that success occurs in even number of tosses                    \[=P(FS)+P(FFFS)+P(FFFFFS)+..........\]                                 \[=pq+{{q}^{3}}p+{{q}^{5}}p+........\]\[=\frac{pq}{1-{{q}^{2}}}=\frac{2}{5}.\]


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