The goal of the riddle is to add just one line to the equation 5+5+5=550 in order to make it true. The solution is to convert 5+5+5=550 to 545+5=550 or 5+545=550.
This is a pretty straightforward puzzle because all that has to be done to transform the plus sign into a four is to do so. Though many individuals can be duped by it because the equation initially appears to be incorrect.
A related conundrum is 3+3+7=350. One of the plus signs will become a four by adding a straight line, solving the problem. These kinds of mental tasks can be found frequently in logic books and collections.