Two bullet trains of equal lengths take 10 seconds and 30 seconds respectively to cross a telegraph post. If the length of each bullet train be 120 metres, in what time (in seconds) will they cross each other travelling in opposite direction? Select the correct option: A) 13 sec . B) 14 sec . C) 12 sec . D) 15 sec . E) 19 sec .

Rationale: "speed of the first bullet train = 120 / 10 m / sec = 12 m / sec . speed of the second bullet train = 120 / 30 m / sec = 4 m / sec . relative speed = ( 12 + 4 ) = 16 m / sec . required time = ( 120 + 120 ) / 16 sec = 15 sec . d"
Answer: D