First you subtract the cost without tax from the cost with the tax: 42.60 - 40 = 2.60; Then you divide the part by the original price before tax: 2.60 ÷ 40 = 0.065; Multiply your answer by 100 to get the actual percent: 0.065 x 100 = 6.5. The percent of the sales tax would be 6.5%.