Respondent shall serve a copy of an answer upon Bar Counsel and file the original with the Executive Attorney within 20 days after service of the petition, unless the time is extended by the Hearing Committee Chair. A respondent who fails to answer within the time provided may request, in writing, permission to file an answer from the Chair of the Hearing Committee to which the matter is assigned if the failure to file an answer was attributable to mistake, inadvertence, surprise or excusable neglect.