Two trains, $120\ \text{m}$ and $80\ \text{m}$ long, move towards each other on parallel tracks at $20\ \text{m/s}$ and $30\ \text{m/s}$. How long do they take to pass each other completely, from the moment their fronts meet?

Fig. 1: Two trains on parallel tracks moving towards each other: one 120 m long at 20 m/s, the other 80 m long at 30 m/s.