Silkie, you're exactly right. Here's how the numbers play out:
00 - 0 = 0
01 - 1 = 0
02 - 2 = 0
...
10 - 1 = 9
11 - 2 = 9
..
19 - 10 = 9
20 - 2 = 18
21 - 3 = 18
..
29 - 11 = 18
30 - 3 = 27
..
99 - 18 = 81
See the pattern?
The answer is always:
x*10 + y - (x+y) = x*9, where x,y is a whole number from 0 to 9.
Therefore, the possible answers are some factor of 9 (0,9,18,27,36,..,81), for a possible total of 10 answers. The orb has a 1 in 10 chance of getting the answer right.
Now, I need some rum. That's too much math for one morning.