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

Answer: Option B

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