20% off $50 = $40. You save $10.
Equation: $50 − (20% × $50) = $50 − $10 = $40.
Another way to read it: you pay 80% of the original price — that's $40.
$40. The discount is $10 and the final price you pay is $40.
You pay $40 (before tax). That's $50 minus a $10 discount.