Question

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

• A. ${10}^{th}$
• B. $9^{th}$
• C. $8^{th}$
• D. None of these

Answer 1

Answer: (B)

$9^{th}$

Clearly, $5\times 2=10$, $10\times 2=20$, $20\times 2=40$, ,.
So, the series is a G.P. in which $a=5$ and $r=2$.
Let $1280$ be the $n^{th}$ term of the series.
Then, $5\times 2^{n-1}=1280\iff 2^{n-1}=256=2^8\iff n-1=8\iff n=9$