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