For an $89.99 item at 30% off, the sale price is 89.99 × 0.70 = $62.99. In Ontario, 13% HST is then added, for a total of $71.18.
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Stacked discounts: watch out
Two discounts in a row don't add up. 20% off followed by an extra 10% off equals 28% off, not 30%. The second discount applies to the already-reduced price: $100 × 0.80 × 0.90 = $72.
Sales tax applies to the sale price
Sales tax is calculated on the price you actually pay, after the discount. A 30% discount therefore also cuts the tax by 30%. Some stores advertise "we pay the tax" promotions, which works out to about an 11.5% discount in Ontario (1 − 1 ÷ 1.13).
Frequently asked questions
How do I calculate 25% off?
Multiply the price by 0.75. A $120 item costs $90 after a 25% discount.
Is sales tax calculated before or after the discount?
After. Tax applies to the sale price, not the original price.
What discount is a "we pay the tax" sale?
In Ontario, about 11.5% off the tax-included price, since the store absorbs the 13% HST. In Quebec, about 13%.