Answer:
Income taxes do not work this way, you generally pay taxes depending on the tax bracket where you fall into. The first two tax brackets for 2019 (taxes due April 2020) for a single filer are:
Tax rate Single filer
10% $0 to $9,700
12% $9,701 to $39,475
Mallory's marginal tax rate = 12%
If Mallory's taxable income after deductions are $12,000, then she will pay:
$9,700 x 10% = $970
($12,000 - $9,700) x 12% = $276
total taxes due = $970 + $276 = $1,246
the effective tax rate = $1,246 / $12,000 = 10.38%