The sales tax rate for a local county is 8.4%. if an item costs $391 (before tax), how much sales tax will be due?

For every 100 dollars, the sales tax would be around 8 dollars and 4 cents. Since there is still 91 dollars you would have to add around 7 dollars and 68 cents. You would go 8 x 3 for 24 plus 7 dollars for 31 dollars and then deal with the change. 4 times 3 is 12 cents plus 68 cents for 80 cents. 

The total would be 31 dollars and 80 cents.

Yeah the answer would be 31 dollars and 80 cents :)