2. Questions Bank
  3. 9th Class
  4. Mental Ability
  5. Series Completion

9th Class Mental Ability Series Completion Question Bank Series Completion

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

    A) 10th                  

    B)        9th

    C) 8th                

    D)        All of these

    E) None of these

    Correct Answer: B

    Solution :

    Explanation Option [b] is correct. Clearly,  \[5\times 2=10,\text{ }10\times 2=20,\text{ }20\times 2-40,\text{ }......\] So, the series is a G.P. in which a = 5 and r = 2. Let 1280 be the nth term of the series. Then, \[5\times {{2}^{n-1}}=1280\Leftrightarrow {{2}^{n-1}}=\,\,256={{2}^{8}}\Leftrightarrow n-1\text{ }=\text{ }8\Leftrightarrow n=9.\]


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