$3720
step-by-step explanation:
taxable income = $31000
as per the 2018-2019 us tax bracket, the tax rate is 12% for an individual whose taxable income is between $9,526 to $38,700.
so, the tax to be paid by him = 12% of 31000
= 0.12 * 31000 [12% converted to decimal as 0.12]
= $3720