Lets take XY as our 2 digit number (where X&Y are the digits)
So XY= 10X + Y
Substract the digits by the number
10X + Y - (X+Y) ==> 9X
So the final answer we get is always divisible by 9
The wordings are given so that all the numbers divisible by 9 get the same word (in ma case it was mouse pad)
So what ever you guess , you will get mouse pad as the answer
eg:
35 - (3+5) = 27 = 3x9
54 - (5+4) = 54 = 5x9
98 - (9+8) = 81 = 9x9
and so on....
So its just simple mathematics...
No mind reading or any magic.,..