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Examveda

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

A. 10th

B. 9th

C. 8th

D. None of these

Answer: Option B

Solution(By Examveda Team)

Clearly, 5 x 2 = 10, 10 x 2 = 20, 20 x 2 = 40,.....
So, the series is a G.P. in which a = 5 and r = 2.
Let 1280 be the rath term of the series.
Then, 5x2n-1 = 1280 ⇔ 2n-1 = 256 = 28 ⇔ n - 1 = 8 ⇔ n = 9.

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