[tex]\it\ell=20\%\ din\ L \Rightarrow \ell=\dfrac{\ \ 20^{(20}}{100}\ \cdot L=\dfrac{1}{5}\cdot L \Rightarrow \ell=\dfrac{L}{5} \Rightarrow L=5\ell\ \ \ \ \ (1)\\ \\ \\ \mathcal{P}=2(L+\ell)=84\bigg|_{:2} \Rightarrow L+\ell=42 \stackrel{(1)}{\Longrightarrow}5\ell+\ell=42 \Rightarrow 6\ell=42 \Rightarrow \ell=7\ m\ \ \ (2)\\ \\ \\ (1),\ (2) \Rightarrow L=5\cdot7=35\ m\\ \\ \\ \mathcal{A}=L\cdot\ell=35\cdot7=245\ m^2[/tex]
