# Which Term of the Series 5, 10, 20, 40, …… is 1280?

- 10
^{th} - 9
^{th} - 8
^{th} - None of these

**Answer: (2) 9th**

**Explanation:- **5, 10, 20, 40, ……………………

Then, 5 x 2 = 10, 10 x 2 = 20, 20 x 2 = 40 …………….

Then, a = 5 and r = 2

Therefore, the given series is G. P. (Geometric Progression),

Let the number of terms be n.

Then, 5 x 2^{n – 1} = 1280

2^{n – 1} = 1280/5

2^{n – 1} = 256

2^{n – 1} = 2^{8}

n – 1 = 8

n = 8 + 1

n = 9

Then, the 9th term is 1280.

Hence, the answer is (2) 9th.