20% off $500 = $400. You save $100.
Equation: $500 − (20% × $500) = $500 − $100 = $400.
Another way to read it: you pay 80% of the original price — that's $400.
$400. The discount is $100 and the final price you pay is $400.
You pay $400 (before tax). That's $500 minus a $100 discount.