When are the absolute value and the opposite of a rational number equal?

Absolute value of X can be treated as a distance on the number line from zero to X, so it has no sign (or it's always positive, never negative).
For example 

but also:


That means that the absolute value of a rational number and its opposite may only be equal, if the rational number is lower than 0 (see the example above). 

The answer for this question is 80