Two trains of equal lengths take 12 sec and 24 sec respectively to cross a telegraph post. If the length of each train be 120 m, in what time will they cross other travelling in opposite direction?
Choose the correct choice from the following choices:  A) 18 sec  B) 12 sec  C) 17 sec  D) 21 sec  E) 16 sec

Rationale: "speed of the first train = 120 / 12 = 10 m / sec . speed of the second train = 120 / 24 = 5 m / sec . relative speed = 10 + 5 = 15 m / sec . required time = ( 120 + 120 ) / 15 = 16 sec . answer : e"
Answer: E