The distance of $5 km$ is in between $A$ and $B$ is covered by $C$ in 4 minute with relative velocity $(v+30)$.
So, $d_{\text {rel }}=v_{\text {rel }} \times t$
$\Rightarrow 5 \,km =(v+30) \times \frac{4}{60}$
$\Rightarrow 75 \,kmh ^{-1}=v+30$
$\Rightarrow v=45 \,kmh ^{-1}$
$[g = 10\,m/{s^2}]$
$\left[ g =10 \,ms ^{-2}\right.$ લો.$]$