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In "Number Series" question, a number series is given with one term missing. Choose the correct alternative that will same pattern and fill in the blank spaces

In following question, a number series is given with one term missing. Choose the correct alternative that will same pattern and fill in the blank spaces.

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

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

Justification:

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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