How can sales tax alter the size of the tip you leave at a restaurant? a. a tip of a given percentage (such as a 20% tip) will be larger if calculated after tax than if calculated before tax. b. the tip becomes part of the bill as a whole, so you have to increase the sales tax by a corresponding amount. c. you do not have to double-calculate a percentage on any purchases, so you should pay for tax or a tip but not both. d. a standard tip is 15% to 20% of the sales tax.