Answer:
Let car
C be moving in opposite direction to A and B, with velocity \[{{\vec{v}}_{c}}\]relative
to ground. Then relative velocity of car C relative to A and B will be
\[{{\vec{v}}_{rel}}={{\vec{v}}_{c}}-\vec{v}\].
As, \[\vec{v}\]is opposite to \[{{\vec{v}}_{c}}\]
So, \[{{\text{v}}_{\text{rel}}}={{\text{v}}_{\text{c}}}-\left(
-\text{3}0
\right)=({{\text{v}}_{\text{c}}}+\text{3}0)\text{km}{{\text{h}}^{-\text{1}}}\]Hence,
time taken by car C to cross the cars A and B will be
\[\text{t }=\frac{s}{{{v}_{rel}}}\]or \[\frac{4}{60}=\frac{5}{{{v}_{c}}+30}\]
or \[{{\text{v}}_{\text{c}}}=\text{45 km}{{\text{h}}^{-\text{1}}}\]
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