As inputs to the truth table 1 and 1 signify that they are both true. The output will depend on what kind of truth table we are talking about, AND, OR, XOR, etc.
Input Output 0 1 1 0
1 x 31 = 31
apparently whenever you can swap the 0's for 1's and 1's for 0's in the truth table and the truth result remains unchanged.
When you are using non-standard definitions of "plus" and "minus", or of "equal".
.....0 10 | 0 01 | 0 1.....0 10 | 0 11 | 1 10 | 11 | 0