10% off $200 = $180. You save $20.
Equation: $200 − (10% × $200) = $200 − $20 = $180.
Another way to read it: you pay 90% of the original price — that's $180.
$180. The discount is $20 and the final price you pay is $180.
You pay $180 (before tax). That's $200 minus a $20 discount.