\(Area\ of\ the\ rectangular:A_{\fbox{ }}=l\times w\\\\A_{\fbox{ }}=\frac{3}{5}\ mi^2;\ l=\frac{7}{8}\ mi;\ w=?\\\\l\times w=\frac{7}{8}w\\\\\frac{7}{8}w=\frac{3}{5}\ \ \ \ |divide\ both\ sides\ by\ \frac{7}{8}\ (multiply\ by\ \frac{8}{7})\\\\w=\frac{3}{5}\times\frac{8}{7}\\\\\boxed{w=\frac{24}{35}\ (mi)}\leftarrow\ answer\)