A is always 1, and C is always 9.
B and D are inversely related with the extremes being 2 and 9.
So all of these answers work:
1) A=1, B=2, C=9, D=9
2) A=1, B=3, C=9, D=8
3) A=1, B=4, C=9, D=7
4) A=1, B=5, C=9, D=6
5) A=1, B=6, C=9, D=5
6) A=1, B=7, C=9, D=4
7) A=1, B=8, C=9, D=3
8) A=1, B=9, C=9, D=2
plug any of these solutions in and you get the right answer...
ex. (ans. 2)
13
+98
------
111
