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