Which number completes the following sequence?

Question

Answer 1

123 = 1+2+3 = 6. So, 123 - 6 = 117
117 = 1+1+7 = 9. So, 117 - 9 = 108
108 = 1+0+8 = 9. So, 108 - 9 = 99
Thus,

99 = 9 + 9 = 18 = 1 + 8 = 9. So, 99 - 9 = 90.
or
99 = 9 + 9 = 18. So, 99 - 18 = 81.

Answer is 90 or 81