Area is w*l and [tex]w=\frac{4l}5[/tex], thus the area is [tex]\frac{4l^2}5[/tex]. Make it equal to 320 and you get [tex]l^2=400[/tex], thus [tex]l=\pm{20}[/tex], the minus one not being meaningful in our context. Thus the length is 20, while the width is 16.