Question

Solve if each letter is a different digit.

ABA
+ CBC
CBCB
-

Answer 1

If two 3-digit numbers add up to a 4-digit number, the leading digit of the answer must be a 1, so C=1.
Now take the tens digit, B+B=1, then
B must be 0 or 5 when we consider the carry of 1 from the units digit.
If A+C=A+1 from the units digit has a carry, then A=9.
Since A+C equal B in both the units and thousand's digits, we conclude that there is not carry in the tens digit (B+B=C), so B=0.
The sum is thus:

 909
+ 101
-------
 1010

Answer 2

1010