Section 151. The financial year of a transportation area shall end on the last day of each calendar year. The cities and towns comprising the area shall, in proportion to their respective interests, as defined in the preceding section, contribute to make up any financial deficit resulting from the operation of the area for any financial year, and the board of assessors of any such city or town, upon receipt from the trustees of a certificate showing the amount so found to be due, shall include said amount in the annual tax levy of the city or town for the year following the said financial year. If there is a surplus from such operation for any financial year, eighty-five per cent thereof shall be distributed by the trustees among the cities and towns comprising the area in proportion to their respective interests, and fifteen per cent thereof shall be held by the trustees to be applied to reduce any deficit in any succeeding year. In reckoning a profit or a deficit, the trustees shall first charge off as depreciation not less than three per cent and not more than five per cent of the book value of the property. Any sum received by the trustees in aid of the operation of the transportation area under any federal or state law shall be credited before reckoning a profit or a deficit.