11.  Interest at the rate of 6% per annum, if payable, shall be calculated on the cost and on the amount which will for the time being remain unpaid, in accordance with such rules as the Minister may prescribe, and the sum of principal and interest shall be apportioned among those proprietors in accordance with the specification mentioned in section 9, and the portion of the cost and interest payable in respect of each such holding shall be paid by the proprietor thereof in 10 equal yearly instalments.