10% off $1,000 = $900. You save $100.
Equation: $1,000 − (10% × $1,000) = $1,000 − $100 = $900.
Another way to read it: you pay 90% of the original price — that's $900.
$900. The discount is $100 and the final price you pay is $900.
You pay $900 (before tax). That's $1,000 minus a $100 discount.