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

10th
9th
8th
None of these

The correct answer is D. None of these.

The series 5, 10, 20, 40, ….. is a geometric series with a common ratio of 2. The general formula for a geometric series is $a_n = a_1 r^{n-1}$, where $a_n$ is the $n$th term, $a_1$ is the first term, $r$ is the common ratio, and $n$ is the number of terms.

In this case, $a_1 = 5$ and $r = 2$. We can use the formula to find the $10$th term:

$a_{10} = 5 \cdot 2^{10-1} = 2^{11} = 2048$

The $9$th term is:

$a_9 = 5 \cdot 2^{9-1} = 2^{10} = 1024$

The $8$th term is:

$a_8 = 5 \cdot 2^{8-1} = 2^{9} = 512$

Therefore, the $10$th term is not 1280, the $9$th term is not 1280, and the $8$th term is not 1280. Therefore, the answer is D. None of these.