What Is The Sum Of 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37 And 39?

Since the numbers are in a series (namely odd numbers below 40), a quick solution is
(1 + 39) x 20 / 2 = 400