$解：(1)t_1∶t_2=\frac {s_1}{v_1}∶\frac {s_2}{v_2}=\frac 49∶\frac 23=2∶3$
$(2)t_1=\frac 25×35\ \text {min}=14\ \text {min}$
$(3)s_{ 1 }=v_{ 1 }t_{ 1 }={ 0.8 }\ \text {m/s}×{ 14×60 }\ \text {s}={ 672 }\ \text {m}$
$s_2=\frac 49×672\ \text {m}=1512\ \text {m}$
$v_{ }=\frac {s_{ }}{t_{ }}=\frac {{ (672+1512) }\ \text {m}}{{ 35×60 }\ \text {s}}={ 1.04 }\ \text {m/s}$
