For the 10% discount, you only have to pay 100% - 10% = 90% of the original price.
For the 20% discount, you only have to pay 100% - 20% = 80% of the original price.
Multiply 0.9 (first discounted price) with 0.8 (second discounted price)
0.9 * 0.8 = 0.72
After these two discounts, you only have to spend 72% of the original price.
That means you got 100% - 72% = 28% total discount.
The single discount is 28% off.
Have an awesome day! :)
