find the sum of the following series 10 + 14 + 18 + 22 + ................ + 104

There should be 102 instead of 104. So, solution for a_n = 102 :
We have, a = 10, d = 14 - 10 = 4, a_n = 102
               a_n = a + (n - 1)d
=>        102 = 10 + (n - 1)4
=>        102 = 10 + 4n - 4
=>        102 = 4n + 6
=>          4n = 96
=>            n = 96/4 = 24
               S_n = n/2 [2a + (n - 1)d]
=>         S₂₄ = 24/2 [2(10) + (24 - 1)4]
=>         S₂₄ = 12 (20 + 92)
=>         S₂₄ = 12 * 112 = 1344
Thus, the sum of the given list of numbers is 1344 when a_n = 102