12th Class Mathematics Sample Paper Mathematics Olympiad - Sample Paper-6

  • question_answer
    A man alternately tosses a coin and throws a die beginning with the coin. The probability that he gets a head in the coin before he gets a 5 or 6 in the dice is

    A)  \[\frac{3}{4}\]                         

    B)  \[\frac{1}{2}\]            

    C)  \[\frac{1}{3}\]                         

    D)  None of these

    Correct Answer: A

    Solution :

    [a] Probability of getting a head in a single toss of a coin \[=1/2\left( =p \right)\] say Probability of getting 5 or 6 in a single throw of a die \[=1/3\left( =q \right)\] say Required probability \[=p+\left( 1-p \right)\left( 1-q \right)p+\left( 1-p \right)\left( 1-q \right)\left( 1-p \right)\left( 1-q \right)p+.....\]\[=p+\left( 1-p \right)\left( 1-q \right)p+{{\left( 1-p \right)}^{2}}{{\left( 1-q \right)}^{2}}p+......\]\[=\frac{p}{1-(1-p)(1-q)}=\frac{1/2}{1-1/2\times 2/3}=\frac{3}{4}\] Hence, option [b] is correct.


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