[tex]t=20\ s\\n=40\\ \lambda=1,2\ m \\ \lambda=\vec{v}\cdot T \ \Rightarrow \vec{v}=\frac{\lambda}{T}=\lambda \cdot \nu \\ \nu=\frac{n}{t}=\frac{40}{20\ s}=2 \ Hz \\ \Rightarrow \vec{v}=1,2 \ m \cdot 2 \ Hz \\ \Rightarrow \boxed{\vec{v}=2,4\ m/s}[/tex]
