There is no precise answer to your question. It is possible to get a 20 mph over ticket reduced to "parking on pavement" depending on what jurisdiction is involved and what your driving record is like. Some courts/prosecutors will not reduce tickets like your to a non-moving (no points) violation, some will. If your seat belt ticket was given at the same time you can probably, under the circumstances, get that "covered" by what whatever plea bargain you can reach with the prosecutor/cop. Be aware that if your ticket was issued in a city where there is a "traffic violatios bureau" (NYC, Buffalo) there is no plea bargaining.
You may want to consider contacting a traffic lawyer to discuss your situation.