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

Answer: Option B

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, 5x2^n-1 = 1280 ⇔ 2^n-1 = 256 = 2⁸ ⇔ n - 1 = 8 ⇔ n = 9.