Which one of these formulas best defines an average tax rate?

Average tax rate $=$ Tax liability $\frac{?}{?}$ Taxable inconte

Average tax rate $=$ Tax liability $\frac{?}{?}$ Total income

Average tax rate $=$ Total taxes paid $\frac{?}{?}$ Net income

Average tax rate $=$ Tax paid on last dollar of income earned$/$$$1$