Short answer: ₹1,000 with 10% then 20% off ends at ₹720 — a saving of ₹280, which is an effective 28%. Adding the percentages would have promised ₹700, and ₹20 of that would never have existed.
The trap in stacked offers
"Extra 20% on the reduced price" is a smaller discount than it sounds, because the base has already shrunk. The table above shows the price after each step, which is the only way to see the real reduction when a sale advertises three discounts in a row.
Worked example
A ₹4,999 jacket at 40% off, with a further 15% coupon: 4,999 → ₹2,999.40 → ₹2,549.49. The effective discount is 49%, not 55%, and the saving is ₹2,449.51.