21. The Minister may redetermine the amount of a real estate tax refund(a) within three years from the day of mailing of the notice provided for in section 18, or
(b) at any time, if the person who has filed the application or the certificate has falsely represented the facts through carelessness or voluntary omission or has committed a fraud in filing the application or certificate or in supplying any other information required by the law or the regulations.