Answer: Always
An example would be 5 + (-5) = 0
Another example is -7 + 7 = 0
The opposite of any positive number is the negative version of that number.
Adding any positive number to its negative counterpart always leads to 0.
A real world example could be "You start on the ground floor. Then you go up 9 floors, and then go down 9 floors in a building, so ultimately you're at the ground floor again". By the term "ground floor", I mean "floor 0".