105 => 1 + 0 + 5 = 6 105 - 6 = 99 99 => 9 + 9 = 18 => 1 + 8 = 9 99 - 9 = 81 Similarly 81 - 9 = 72 The series ends with 0.
It follows below logic 105 - 6 = 99 1+0+5 = 6 99 - 9 = 90 9 + 9 = 18 18 = 1 + 8 = 9 90 - 9 = 81 9 + 0 = 9 81 - ? = ? 8 + 1 = 9 Then, 81 - 9 = 72