Two trains of equal lengths take 15 sec and 20 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?	Choices.
A) 16 sec  B) 12 sec  C) 17 sec  D) 21 sec  E) 23 sec

Rationale: "speed of the first train = 120 / 15 = 8 m / sec . speed of the second train = 120 / 20 = 6 m / sec . relative speed = 8 + 6 = 14 m / sec . required time = ( 120 + 120 ) / 14 = 17 sec . answer : c"
Answer: C