Two trains of equal lengths take 10 sec and 15 sec, respectively, to cross a telegraph post. If the length of each train is 120 m, in what time will they cross each other, travelling in opposite directions? Choose the correct choice from the following choices:  A) 17 sec  B) 12 sec  C) 18 sec  D) 19 sec  E) 52 sec

Rationale: speed of the first train = 120 / 10 = 12 m / sec . speed of the second train = 120 / 5 = 8 m / sec . relative speed = 12 + 8 = 20 m / sec . required time = ( 120 + 120 ) / 20 = 12 sec . answer : b
Answer: B