10% off $100 = $90. You save $10.
Equation: $100 − (10% × $100) = $100 − $10 = $90.
Another way to read it: you pay 90% of the original price — that's $90.
$90. The discount is $10 and the final price you pay is $90.
You pay $90 (before tax). That's $100 minus a $10 discount.