What is the sum of the first 20 terms of the series 3 + 14 + 25 + 25 + 36 +

Do you mean to have the number 25 repeated?

Last term=3 + 11*(20 - 1)
                =3 + (11*19)
                =3 + 209
                =212
Sum =[F + L] / 2 * N, where F=First term, L=Last term, N = Number of terms.
Sum =[3 + 212] / 2 * 20
         =215 / 2 * 20
         =107.50 * 20
         =2,150