How to calculate a discount
The sale price is the original price minus the discount. In numbers: Sale price = Original × (1 − discount ÷ 100). The amount you save is the original price minus the sale price.
Stacked (double) discounts
When a store offers "an extra 10% off already-reduced items", the second discount applies to the first sale price, not the original. That's why two discounts always combine to less than their simple sum.
How do I find the original price from a sale price?
Divide the sale price by (1 − discount ÷ 100). e.g. a £60 item at 25% off originally cost 60 ÷ 0.75 = £80.