Two bullet train s 140 m and 180 m long run at the speed of 60 km/hr and 40 km/hr respectively in opposite directions on parallel tracks. The time (in seconds) which they take to cross each other, is:
Choose the correct choice from the following choices.
A) 15.8 sec .  B) 11.52 sec .  C) 11.8 sec .  D) 10.8 sec .  E) 08.8 sec .

Rationale: "relative speed = ( 60 + 40 ) km / hr = 100 x 5 / 18 = 250 / 9 m / sec . distance covered in crossing each other = ( 140 + 180 ) m = 320 m . required time = 320 x 9 / 250 = 11.52 sec . answer b"
Answer: B