The area of the stained glass is given as 20ft².
Let's start by finding the length of one side of the square stained glass. Since the area of a square is given by the formula A = s^2, where s is the length of one side of the square, we have:
s^2 = A
s^2 = 20ft²
s = sqrt(20)ft
s = 2sqrt(5)ft
Now, we can use the given equation p^2 = 5A to find the perimeter p of the square, by substituting the value of A:
p^2 = 5A
p^2 = 5(20ft²)
p^2 = 100ft²
p = sqrt(100ft²)
p = 10ft
Therefore, the perimeter of the square stained glass is 10 feet.