The equation 5+5+5=550 is a riddle where the directions are to make the equation true by adding only one single line. The answer is to change the equation from 5+5+5=550 to 545+5=550 or 5+545=550.
This is a very simple riddle because the plus sign just has to be reworked into a 4 to make the solution work. It can fool many people though because the equation at first just looks erroneous.
A similar riddle could be 3+3+7=350. Adding a straight line to one of the plus signs will change it to a four, fixing the equation. These types of brain teasers are common in logic books and collections of mind exercises.