[tex]\it \ell=20\%\ din\ L=\dfrac{\ \ 20^{(20}}{100}\cdot L= \dfrac{1}{5}\cdot L=\dfrac{L}{5}\ \ \ \ \ \ (1)\\ \\ \\ \mathcal{P}=2(L+\ell)=84\bigg|_{:2} \Rightarrow L+\ell=42 \stackrel{(1)}{\Longrightarrow} L+\dfrac{L}{5}=42\bigg|_{\cdot5}\Rightarrow \\ \\ \\ \Rightarrow 5L+L=5\cdot42\Rightarrow 6L=5\cdot42\bigg|_{:6}\Rightarrow L=5\cdot7=35\ m\ \ \ \ \ (2)\\ \\ \\ \ell=\dfrac{35}{5}=7\ m\\ \\ \\ \mathcal{A}=L\cdot\ell =35\cdot7=245\ m^2[/tex]
