Area are all sides multiplied the formula can be found online
You'd simply have to find 20% of $40. To find the percent of a number, you would change the percent into an actual decimal/number, and then multiply them both. So the percent would become 0.20.
40 × 0.20 = 8
So now you would subtract 8 from the cost of 40.
40 - 8 = 32.
The shirt costs $32.
Not enough information to "solve for X and Y". it can be a number of things; 1 and 1. 0 and 2. -1 and 3. There is no solving unless there is more information