解：?$(1)v_1=\frac {s_1}{t_1}=\frac {{ 300 }\mathrm {km}}{5\ \mathrm {h}}={ 60 }\mathrm {km/h}$?
?$ (2)v_2=\frac {s_2}{t_2}=\frac {{ 300 }\mathrm {km}}{{2.5 }\ \mathrm {h}}={ 120 }\mathrm {km/h}$?
?$ (3)?t=t_1-t_2=5\ \mathrm {h}-2.5\ \mathrm {h}=2.5\ \mathrm {h}$?
