This is a proportion problem. When we equal two ratios, we end up with a proportion. In this case, we have:
[tex]\frac{L}{W}=\frac{7}{3}[/tex]Where L is for length and W for width.
We also have that the length is 20, then, we have:
[tex]\frac{L}{W}=\frac{7}{3}\rightarrow\frac{W}{L}=\frac{3}{7}\rightarrow W=\frac{3\cdot L}{7}\rightarrow W=\frac{3\cdot20}{7}[/tex][tex]W=\frac{60}{7},W=8.57142\ldots[/tex]Then, the width is W = 60/7 (or about W = 8.5714 feet).