Two trains $120m$ and $80m$ in length are running in opposite directions with velocities $42km/hr$ and $30km/hr$ respectively. In what time will they completely pass each other.

The total distance to be travelled by the trains = $120+80=200m$

We know that, when the trains are traveling in the opposite direction then the relative velocity will be the sum of the velocities of the running body.

So, $Relativevelocity=42+30=72km/hr$

Now, converting it into $m/s$

So, $$\begin{gathered} 72km/hr=72\times \frac{1000}{60\times 60}m/s \\ 72lm/hr=20m/s \end{gathered}$$

Now, we know that

$$\begin{gathered} Time=\frac{Dis\tan ce}{Speed} \\ Time=\frac{200}{20} \\ Time=10sec \end{gathered}$$

Hence, the time taken to pass each other completely will be $10sec$.