The area of a rectangle is length times width.
The given values are area = 9/20, length is x, and width is x/5. So we can put this into one whole formula.
9/20 = x(x/5)
0.45 = (x^2)/5
2.25 = x^2
x = square root of 2.25 = 1.5
So the length is 1.5 miles and width is 1.5/5 miles, or 0.3 miles.
The dimensions are 1.5x0.3