You don't subtract the percentage of the tip (18%) from the percentage of the discount (20%). Too many posters do that and then complain that it's only a 2% discount, which isn't the case. To calculate correctly, subtract the amount you would be paying plus tip without the discount, from the amount you would be paying plus 18% tip with the discount (as Cheshire Figment has shown above). This does assume that you intend to tip. If you don't intend to tip at all, then the complaint that there are no savings may be justified.