Answer: 12%
It is important to note that the coupon is applied first THEN the fee is applied.
First, we find the percent of the price that will be paid after the coupon is applied when converted to a decimal.
100%-20%= 80%= 0.8
Now, we multiple this decimal by the original price.
75(0.8)= 60
Then, we find the percent of the price that will be paid after the fee is added when converted to a decimal.
100%+10%= 110%= 1.1
Once we found the decimal we multiply it by the price after the coupon is applied.
60(1.1)=66
66 is 88% of 75, the original price or 100%.
Subtract new price from original value.
100%-88%= 12%
If you have any questions feel free to ask. :)