\(\left\{{}\begin{matrix}v_1=\dfrac{s_1}{t_1}=\dfrac{180}{45}=4\left(\dfrac{m}{s}\right)\\v_2=\dfrac{s_2}{t_2}=\dfrac{100}{30}=\dfrac{1}{3}\left(\dfrac{m}{s}\right)\\v_{tb}=\dfrac{s_1+s_2}{t_1+t_2}=\dfrac{180+100}{45+30}=\dfrac{56}{15}\left(\dfrac{m}{s}\right)\end{matrix}\right.\)