Two trains 108 m and 112 m in length are running towards each other on parallel lines at a speed of 45 km/hr. and 54 kmph respectively. How long will it take them to cross each other after they meet ?

Trains are running towards each other, so relative speed = 45 + 54 = 99 km/hr = 99 × (5/18) = 27.5 m/s. Total distance to cover = sum of lengths = 108 + 112 = 220 m. Time = Distance/Speed = 220/27.5 = 8 seconds. Option A (6 sec), C (7 sec), and D (10 sec) are incorrect. The key is understanding that 'after they meet' means the time to completely cross each other, which uses relative speed and total length.