Two trains A and B leave Delhi for Hyderabad at 7:00 a.m. and 7:50 a.m. on the same day and travel at 80 kmph and 100 kmph respectively. After how many kilometers from Delhi will the two trains be together?

Train A leaves at 7:00 a.m. at 80 kmph. Train B leaves 50 minutes (5/6 hour) later at 100 kmph. In 5/6 hour, train A covers 80 × 5/6 = 400/6 = 200/3 km, giving train B a head start to catch up. Relative speed = 100 - 80 = 20 kmph. Time for B to catch A = (200/3)/20 = 10/3 hours. Distance from Delhi where they meet = 100 × 10/3 = 1000/3 km.