Subject to the provisions of articles X.1 ( b) and XVI, a participant who, before reaching the age of sixty, has, in the opinion of the Joint Staff Pension Board, become unable to perform his duties owing to serious physical or mental impairment, shall be entitled, subject to article IX, while such disability continues, to a disability benefit payable in the same manner as a retirement benefit and equal to nine-tenths of one-fifty-fifth of his final average remuneration multiplied by the number of years of his contributory service not exceeding thirty years. This disability benefit shall not be less than the smaller of: