Section 10B. On or before August tenth next following, a city or town aggrieved by the equalized valuation established by the commissioner under sections nine and ten A may appeal the determination of the commissioner to the appellate tax board. The appellant shall have the burden of proving that the equalized valuation of such city or town is substantially different from the commissioner’s determination. Every such appeal shall be decided by the board not later than January twentieth of the year next following the year in which it is filed. If the board fails to act upon an appeal within said time, it shall be deemed to be a denial of such appeal. The decision of the board shall be final.