Given: Speed of train A = 80 km/hr. Train B start running after 3 hr. of train A starting In 6 hr. after starting of train B, Both trains cross each other. Distance = 720 km. Concept: When two objects running same direction with x km/hr. and y km/hr and if x > y than relative speed = (x – y) km/hr. Distance = Relative speed × time Calculation: In first 3 hr. train A cover distance = 80 × 3 ⇒ 240 km. When train B start running, Train A 240 km ahead and for cross each other it is required that speed of train B more than the speed of train B. Let speed of train B = x km/hr. Relative speed = (x – 80) km/hr. After 6 hr. train A and train B maintain balance distance of 240 km. Distance/Relative speed = time 240/(x – 80) = 6 ⇒ 6x = 720 ⇒ x =120 km/hr. Speed of train B = 120 km/hr. Train A take time to cover distance of 720 km. with speed of 80 km. = 720/80 ⇒ 9 hr. Train B take time to cover distance of 720 km. with speed of 120 km. = 720/120 ⇒ 6 hr. (Time taken by train A to cover 720 km – Time taken by train B to cover 720 km) = 9 – 6 ⇒ 3 hr. ∴ Train A takes 3 hours more than time taken by train B to cover distance of 720 km.