Two bullet trains of equal lengths take 10 seconds and 20 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:
A) 11 sec . B) 15 sec . C) 13.3 sec . D) 17 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 / 20 m / sec = 6 m / sec . relative speed = ( 12 + 6 ) = 18 m / sec . required time = ( 120 + 120 ) / 18 sec = 13.3 sec . c"
Answer: C