The amount you save is the original price × discount ÷ 100. The sale price is the original price minus that amount, or equivalently the original price × (1 − discount ÷ 100). If sales tax applies, it is normally charged on the discounted price, so the final price is the sale price × (1 + tax ÷ 100).
Example: an $80 item at 25% off saves $20, so the sale price is $60. With 8% sales tax the final price is $64.80.
Multiply the price by 0.8. For $50 that gives $40. In your head, take 10% (move the decimal one place), double it, and subtract.
They do not add up to 30%. Apply them one after the other: the price is multiplied by 0.8 and then by 0.9, which is 0.72, so the total discount is 28%.
In most places sales tax is charged on the price after a store discount. Manufacturer coupons can be treated differently, so check the receipt.
No, everything is calculated in your browser.
See also: Percentage Calculator · Tip Calculator · Loan Calculator