Question

Find the sum of the first 16 terms of the geometric series 1/5 + 1/20 + 1/80...

Answer 1

\[S_n = a \frac{ 1 - r^n }{ 1 - r }\]

where 'a' is the first term that you have (1/5)
'n' is the number of terms you want to sum (16)
'r' is the common ratio (how you get from one term to the next)

so what is your common ratio here?

Answer 2

*hint...what do you multiply by to get from 1/5 to 1/20?