An easy way to do this one is to take the number of sides (n) and add 1, then divide by 2. You'll multiply the result by number of sides20 and that'll be your answer. So ((n + 1) / 2) * n.
For a 10 sided die that's:
((10 + 1) / 2) * 10 =
(11 / 2) * 10 =
5.5 * 10 =
55
Applying the formula to the original question we get:
((20 + 1) / 2) * 20 =
(21 / 2) * 20 =
10.5 * 20 =
210
It even works for large or odd numbers!
((117 + 1) / 2) * 117 =
(118 / 2) * 117 =
59 * 117 =
6,903
