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
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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.
