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