Σ means "the sum of"
In this case it means "the sum from when r=1 to when r=20"
So you set r=1 first and add it to what you get when r=2... etc... up to r=20.
Doing this by hand would take a long time so there are some tricks.
It turns out that it is just the same as:
Σ 5 - Σ 2r = Σ 5 + 2Σ r
(The r=1 and 20 should still be there, just I cant put them in on a post.)
Well 5 stays the same no matter what r is, so Σ 5 = 5x20 = 100
Σr is just the sum of all the numbers from 1 to 20, which I believe = .5 x 20 x 21 = 210.
Therefore your answer should be 100 + 420 = 520.