Explanation:

To solve this question, we can apply a short trick approach;
Net% effect = ( x + y + xy /100)%
Increase or decrease, according to the +ve or –ve sign respectively.
Given;
First discount = x = –10%
Second discount = y = – 10%
By the short trick approach, we get
= [- 10 - 10 + (-10 x -10) /100 ] = - 19%