Question

What single discount is equivalent to successive discounts of 10% and 20%?

Answer 1

Let's start with $100 to make the problem easy.
10% of this is $10, so subtracting that from $100 we get $90.
20% of $90 is $18, so subtract that from $90 and we get $72.

$72 is 72% of $100, so the combined discount is 28%

Answer 2

The general formula would be (where C is the combined discount percent, F is the first discount percent, and S is the second discount percent):

C = F + S - F*S