A b

B.C.

-

B C D

And BCD, which means carry. When two numbers are added, the carry can only be 1, so B= 1.

Then, if one bit of B+C has no carry, ten bits of A can only be 9, and A+B can only carry (where B= 1).

If the B+C of the single digit has a carry, the a of the ten digit may be 9 or 8.

When B+C has a carry, if A=8, then C=0, then B+C = 1, there can be no carry, which is irrelevant.

When B+C has a carry, if A=9, then C= 1, and then B+C =2, there can be no carry, which is irrelevant.

So A =9, B+C has no carry, then C = 0, D =B+C = 1.

Therefore, 91+10 =101