Two bullet trains of equal lengths take 10 seconds and 12 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?
Choose the correct choice from the following answers.
A) 11 sec . B) 15 sec . C) 12 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 / 12 m / sec = 10 m / sec . relative speed = ( 12 + 10 ) = 22 m / sec . required time = ( 120 + 120 ) / 22 sec = 11 sec . a"
Answer: A