(a) For taxable years ending on or before December thirty-first, nineteen hundred seventy, the tax imposed by section two shall be determined in accordance with the following table: If the city taxable income is: The tax is: Not over $1,000 0.4% of the city taxable income Over $1,000 but not $4 plus 0.6% of excess over $1,000 over $3,000 Over $3,000 but not $16 plus 0.8% of excess over $3,000 over $6,000 Over $6,000 but not $40 plus 1.0% of excess over $6,000 over $10,000 Over $10,000 but not $80 plus 1.2% of excess over $10,000 over $15,000 Over $15,000 but not $140 plus 1.4% of excess over $15,000 over $20,000 Over $20,000 but not $210 plus 1.6% of excess over $20,000 over $25,000 Over $25,000 but not $290 plus 1.8% of excess over $25,000 over $30,000 Over $30,000 $380 plus 2.0% of excess over $30,000

(b) For taxable years beginning on or after January first, nineteen hundred seventy-one and ending on or before December thirty-first, nineteen hundred seventy-five, and taxable years beginning in nineteen hundred seventy-five and ending in nineteen hundred seventy-six, the tax imposed by section two shall be determined in accordance with the following table: If the city taxable income is: The tax is: Not over $1,000 0.7% of the city taxable income Over $1,000 but not $7 plus 1.1% of excess over $1,000 over $3,000 Over $3,000 but not $29 plus 1.4% of excess over $3,000 over $6,000 Over $6,000 but not $71 plus 1.8% of excess over $6,000 over $10,000 Over $10,000 but not $143 plus 2.1% of excess over $10,000 over $15,000 Over $15,000 but not $248 plus 2.5% of excess over $15,000 over $20,000 Over $20,000 but not $373 plus 2.8% of excess over $20,000 over $25,000 Over $25,000 but not $513 plus 3.2% of excess over $25,000 over $30,000 Over $30,000 $673 plus 3.5% of excess over $30,000

(c) For each taxable year beginning in nineteen hundred seventy and ending in nineteen hundred seventy-one, two tentative taxes shall be computed, the first as provided in subdivision (a) and the second as provided in subdivision (b), and the tax for each such year shall be the sum of that proportion of each tentative tax which the number of days in nineteen hundred seventy and the number of days in nineteen hundred seventy-one, respectively, bears to the number of days in the entire taxable year.