Area = length x width
But width = 4/5 x length
so 320 = length x 4/5length
Rearranged, 320 = 4/5(length)(length)
length x length = length^2
Divide both sides by 4/5
400 = (length)^2
Square root both sides
20 = length
So now you know the length, you can calculate the width.
Width = 4/5 x length
= 4/5 x 20
= 16
Length = 20ft, Width = 16ft