t_{1} = \frac{2 l}{v_{2}} = \frac{8 m}{20 \frac{m}{s}} = 0, 4 s

${t_1 = \dfrac{2l}{v_2} = \frac{8 \,\mathrm{m}}{20 \,\mathrm{\frac{m}{s}}} = 0,4\,\mathrm{s}}$