20% off $150 = $120. You save $30.
Equation: $150 − (20% × $150) = $150 − $30 = $120.
Another way to read it: you pay 80% of the original price — that's $120.
$120. The discount is $30 and the final price you pay is $120.
You pay $120 (before tax). That's $150 minus a $30 discount.