\(\left\{{}\begin{matrix}v_1=\dfrac{s_1}{t_1}=\dfrac{120}{30}=4\left(\dfrac{m}{s}\right)\\v_2=\dfrac{s_2}{t_2}=\dfrac{75}{15}=5\left(\dfrac{m}{s}\right)\\v_{tb}=\dfrac{s_1+s_2}{t_1+t_2}=\dfrac{120+75}{30+15}=\dfrac{13}{3}\left(\dfrac{m}{s}\right)\end{matrix}\right.\)