Which term of the G.P. 5,10,20,40 ...is 1280?
A) Eighth B) Ninth C) Tenth D) Eleventh

• Clearly, 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 * 2n - 1 = 1280
  
  ⇒
  
  2
  
  n - 1
  
  = 256 = 28.
  .'. n - 1 = 8 or n = 9.
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