[tex]n=20 \\ t=50\ s \\ \vec{g}=9,8\ m/s^2 \\ T=2\pi\sqrt{\frac{l}{\vec{g}}}=\frac{t}{n}=\frac{50 \ s}{20}=2,5 \ s \\ \Rightarrow \sqrt{\frac{l}{\vec{g}}}=\frac{T}{2\pi}=\frac{2,5 \ s}{2\pi} \\ \Rightarrow \frac{l}{\vec{g}}=(\frac{2,5\ s}{2\pi})^2=\frac{6,25\ s^2}{4\pi^2} \\ \Rightarrow l=9,8 \ m/s^2 \cdot \frac{6,25 \ s^2}{2 \cdot 3,14}=\frac{61,25 \ m}{6,28} \\ \Rightarrow \boxed{l=9,75 \ m}[/tex]
