20 mills means that you have to pay $20 for every 1,000 of value.
The standard computation of tax based on the millage rate is to multiply the rate to the value of the property and divide the product by 1,000.
20 x $80,000 = $1,600,000
$1,600,000 ÷ 1,000 = $1,600
Harriet Walker should pay C.) $1,600 for property tax.