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

Aptitude Quizzes Mathematics Aptitude Test

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

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

Correct Answer is Option C):
9th

Explanation:
Clearly, 5x2 = 10,10x2 = 20, 20x2 =40 So. the series is a G.P. in which a = 5 and r = 2. Let the number of terms be n. Then, 5x2(power n-1) = 1280 => 2(power n-1) = 256 = 2(power n). :. n—1 = 8 or n = 9