To calculate tax you must put the percentage in decimal form. Ex: 0.09 You will then multiply it by the amount of money also in decimal form. 9.00 x 0.09 = 0.81 So 0.81 is 9% of $9. You will then add 0.81 to 9.00 for a total of 9.81, formerly written as $9.81 :)