Answer:
x = 5 feet
Explanation:
Let L and W be the length and width of the pool.
L = 20 feet
W = 20-x feet
L*W = 300 ft^2
To find x, use the area equation, use 20 feet for L and substitute the value of W as defined above:
L*W = 300 ft^2
(20 ft)*(20ft-x) = 300 ft^2
400 ft^2 - 20x = 300 ft^2
-20x = -100
x = 5 feet