Question

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

Answer 1

9th term...it's geometric progression ,use ar^(n-1)

Answer 2

5 , 10 , 20 , 40 form a G . P

Here, r = 10/5 = 2.

Let 1280 be nth term .

We know that,

a* r^n-1 = nth term .

1280 = 5 * 2 ^ n - 1

256 = 2^n-1

2^8 = 2^n-1

n - 1 = 8

n = 9 .

Therefore, 1280 is 9th term.