This is a geometric progression with first term a=5 and common ratio r=2. The nth term is $ar^{n-1}$. So, $5 \times 2^{n-1} = 1280 Rightarrow 2^{n-1} = 256 = 2^8$. Thus, n-1=8, so n=9. The answer key says 8th, which is a common off-by-one error.