Here is one possible answer:

1) You purchase a item for 3.99 dollars and pay 7.5 % (percent) in tax. How much is tax and the final retail price including tax?

Firstly, if the tax is expressed in percent, divide the tax rate by 100. You can do this by simply moving the decimal point two spaces to the left. In this example, we do 7.5/100 = 0.075

Now, find the tax value by multiplying tax rate by the item value:

tax = 3.99 × 0.075

tax = 0.29925

Finally, add tax to the before tax price to get the final price including tax:

The final price including tax = 3.99 + 0.29925 = **4.28925**

**Sales tax Formula (Final Price)**

Final price including tax = before tax price × (1 + ^{tax rate (%)} / _{100}) or,

if the tax is alread expressed in decimal,

Final price including tax = before tax price × (1 + tax rate)

2) You buy a item for 4.28925 dollars after tax was added. You know that the applicable tax rate is 7.5%. How much is the before tax price?

Divide the percentage added to the original by 100. Here, a sales tax of 7.5 percent was added to the bill to make it 4.28925. So, divide 7.5 by 100 to get 0.075.

Add one to the percentage, expressed as a decimal. In this example, add one to 0.075 to get 1.075.

Divide the final amount by the result of the previous step to find the original amount before the percentage was added. In this example, divide 4.28925 by 1.075 to find the amount before the sales tax was added, which equals 3.99.

**Sales tax Formula (Before Tax Price)**

Before Tax Price ($) = Final price / (1 + ^{tax rate(%)} / _{100}) or,

if the tax is alread expressed in decimal,

Before Tax Price ($) = ^{Final price} / _{(1 + tax rate)}

Reference:

http://coolconversion.com/finance/sales-tax-calculator/