10% off $20 = $18. You save $2.
Equation: $20 − (10% × $20) = $20 − $2 = $18.
Another way to read it: you pay 90% of the original price — that's $18.
$18. The discount is $2 and the final price you pay is $18.
You pay $18 (before tax). That's $20 minus a $2 discount.