Convert the % numbers to decimals: 6.25% = 0.0625, 20% = 0.2 Now, the bill of $28.35 must add 6.25% of 28.35 for tax and 20% of 28.35 for tip. So, to use an equation: Total cost = 28.35 + (28.35)(0.0625) + (28.35)(0.2) = 28.35 + 1.77 + 5.67 = $35.79.