The tax levied and imposed by Section 40-18-2 shall be computed as follows:
(1) For a single person, head of family, or married persons filing separate returns:
a. Two percent of taxable income not in excess of five hundred dollars ($500).
b. Four percent of taxable income in excess of five hundred dollars ($500) and not in excess of three thousand dollars ($3,000).
c. Five percent of taxable income in excess of three thousand dollars ($3,000).
(2) For married persons filing a joint return:
a. Two percent of taxable income not in excess of one thousand dollars ($1,000).
b. Four percent of taxable income in excess of one thousand dollars ($1,000) and not in excess of six thousand dollars ($6,000).
c. Five percent of taxable income in excess of six thousand dollars ($6,000).