解：$(1)t_{ 1 }=\frac {s_{ 1 }}{v_{ 1}}=\frac {{ 90 }\ \text {km}}{{ 60 }\ \text {km/h}}={ 1.5 }\ \text {h}$
$t_{ 2 }=\frac {s_{ 2}}{v_{2 }}=\frac {{ 30 }\ \text {km}}{{ 40 }\ \text {km/h}}={ 0.75 }\ \text {h}$
$t=t_1+t_2=2.25\ \text {h}$
$(2)v_{ }=\frac {s_{ }}{t_{ }}=\frac {{ 120 }\ \text {km}}{{ 2.25 }\ \text {h}}={ 53.33 }\ \text {km/h}$
