Logic for custom number system...

Question

I need to create a logic that works for a number system..

means if A=1, B=2, C=3, D=4,E=5, F=6; G=7,H=8,I=9, O=0;

 if i give input as A+B, then result should be come as C, not 3,

or if i give F+G then output should be  AC;

 

 

if any one  have some  idea about it,, post that..