Section 30. The amount which the county commissioners of each county, except Suffolk and Nantucket, shall levy as the county tax shall be as voted by the county commissioners and approved by the advisory board on county expenditures, and as computed by adding together the amounts of the approved annual budget and of any approved supplementary budget, so far as the money therefore is to be raised by taxation, and deducting therefrom the probable receipts from all sources, except loans, and so much of the unexpended balance from the county treasury at the closing of the treasurer's books for the previous fiscal year as is in excess of ten per cent of the tax levy for said year; provided, however, that the county commissioners shall deduct the unexpended balance from the previous fiscal year from the tax voted by the county commissioners and approved by the advisory board prior to the apportionment and assessment of said tax among and upon the several cities and towns. The county commissioners shall also levy annually as the county tax a sum sufficient to meet the debt and interest maturing in that year, if no other provision therefor has been made.