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Railways R.R.C. (Patna) Solved Paper Held on 1st Shift 16-11-2014

  • question_answer
    Which term of the series 5,10,20,40,... is 1280?

    A) 10th                             

    B) 9th

    C) 8th                               

    D) 11th

    Correct Answer: B

    Solution :

    \[1280=5\times {{(2)}^{n-1}}\] \[256={{2}^{n-1}}\] \[{{2}^{8}}={{2}^{n-1}}\] \[\therefore \]      \[n-1=8\] \[\therefore \]      \[n=9th\]


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