[tex]l=1 \ m \\ t=3 \ min =180 \ s \\ g=9,8 \ m/s^2 \\ T=2\pi \sqrt{\frac{l}{g}}=2\pi \sqrt{\frac{1 \ m}{9,8 \ m/s^2}}=2\pi \cdot \frac{1}{3,13} \ s\approx 2 \ s; \\ T=\frac{t}{n} \Rightarrow n=\frac{t}{T}=\frac{180 \ s}{2 \ s} \\ \Rightarrow \boxed{n=90 \ oscilatii}[/tex]
