解：$(1)t=09$：$53-09$：$15=48 \mathrm {\mathrm {min}}=0.8\ \mathrm {h}$
$v=\frac {s}{t}=\frac {84\ \mathrm {km}}{0.8\ \mathrm {h}}=105\ \mathrm {km/h}$
$(2)t'=10$：$35-09$：$53=42 \mathrm {\mathrm {min}}=0.7\ \mathrm {h}$
$s'=v't'=129\ \mathrm {km/h}×0.7\ \mathrm {h}=90.3\ \mathrm {km}$
