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

A. 10th
B. 9th
C. 8th
D. None of these

• B. 9th term
  
  Its like 5 * 2 = 10, 10 * 2 = 20, 20 * 2 = 40,...
  
  So, the series is a G.P. in which a = 5 and r = 2.
  Let the number of terms be n.
  Then, 5 * 2^n - 1 = 1280
  
  2^n - 1 = 256 => 2^8.
  n - 1 = 8
  n = 9.
• 12 years agoHelpfull: Yes(2) No(0)