Adding tax to a purchase can be accomplished with basic mathematics that include multiplication, percentages, decimals and addition. It is only necessary to know the sales tax in a given area as a percentage of the total purchase.Continue Reading
Begin by converting the percentage of sales tax into a decimal number. For example, 10 percent sales tax becomes 0.10.
Calculate the tax on the purchase by multiplying the purchase price by the sales tax. A car that costs $20,000 and has 10 percent sales tax would be taxed for $2,000.
Add the number calculated sales tax to the purchase price to find the total price with tax. The $20,000 car with $2,000 tax would cost $22,000.