Two cars are moving in the same direction with the same speed $30\text{km/hr}$. They are separated by a distance of 5 km, the speed of a car moving in the opposite direction if it meets these two cars at an interval of 4 minutes, will be 

The two cars (say A and B) are moving with same velocity, the relative velocity of one (say B) with respect to the other

$\text{A},{\overrightarrow{v}}_{\text{BA}}={\overrightarrow{v}}_{\text{B}}-{\overrightarrow{v}}_{\text{A}}=\text{v}-\text{v}=0$

So the relative separation between them $\left(-5\text{km}\right)$ always remains the same.

Now if the velocity of car (say C) moving in opposite direction to A and B, is ${\overrightarrow{v}}_C$ relative to ground then the velocity of car C relative to A and B will be  ${\overrightarrow{v}}_{rel}={\overrightarrow{v}}_C-\overrightarrow{v}$ 

But as $\overrightarrow{v}$ is opposite to ${\overrightarrow{v}}_C$ ,so

${\overrightarrow{v}}_{rel}-{\overrightarrow{v}}_C-\left(-30\right)=\left({\overrightarrow{v}}_C+30\right)\text{km/hr}$So, the time taken by it to cross the cars A and B

$\text{t}=\frac{\text{d}}{{\text{v}}_{\text{rel}}}$

$\Rightarrow \frac{4}{60}=\frac{5}{v_C+30}$

$\Rightarrow {\text{v}}_{\text{C}}=45\text{\hspace{0.17em}km/hr}$