Based on the amounts of taxable income below, compute the federal income tax payable in 2018 on each amount assuming the taxpayers are married filing a joint return. Also, for each amount of taxable income, compute the average tax rate and the marginal tax rate.

a. Taxable income of $30,000

b. Taxable income of $100,000

c. Taxable income of $375,000

d. Taxable income of $700,000