Answer
$\displaystyle \sum_{n=1}^{20}5n$
Work Step by Step
From the first three terms, we note the pattern
n-th term = $5n$, for n=1, 2, 3.
Since the last term is $100=5(20)$, we conclude that it is the 20th term of the sum.
Summation notation: $\quad \displaystyle \sum_{n=1}^{20}5n$
