The distance of \(5 km\) is in between \(A\) and \(B\) is covered by \(C\) in 4 minute with relative velocity \((v+30)\).
So, \(d_{\text {rel }}=v_{\text {rel }} \times t\)
\(\Rightarrow 5 \,km =(v+30) \times \frac{4}{60}\)
\(\Rightarrow 75 \,kmh ^{-1}=v+30\)
\(\Rightarrow v=45 \,kmh ^{-1}\)
($g = 9.8\,m/{s^2}$)