haha you lot. You are supposed to always get 9

Reason:

Pick a number x such that 1 < x < 9
then multiply by 3 so you have 3x
then add 3 so you have 3x + 3 which is equal to 3(x+1)
multiply by 3 so you have 9(x+1)

So essentially the two digit number you have is in the multiplication number of 9
9, 18, 27, 36, 45, 54, .... so if you add the two digits you always get 9 =))
*nerd mode on*