15% off $100 = $85. You save $15.
Equation: $100 − (15% × $100) = $100 − $15 = $85.
Another way to read it: you pay 85% of the original price — that's $85.
$85. The discount is $15 and the final price you pay is $85.
You pay $85 (before tax). That's $100 minus a $15 discount.