[tex]m=40\:g=0,04\:kg\\K=0,02\frac{N}{mm}\\F_e=K\Delta l,\:\:F_e=G\implies \Delta l=\frac{G}{K}\\\Delta l=v\Delta t\implies v=\frac{\Delta l}{\Delta t}\\\implies v=\frac{\frac{G}{K}}{\Delta t}=\frac{mg}{K\Delta t}\\\text{Numeric, } v=\frac{0,4N}{0,02N/mm\cdot 0,5s}=40\frac{mm}{s}[/tex]