Two trains of equal lengths take 10 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? Answers:
A) 16 sec  B) 13 sec  C) 17 sec  D) 21 sec  E) 23 sec

Rationale: "speed of the first train = 120 / 10 = 12 m / sec . speed of the second train = 120 / 20 = 6 m / sec . relative speed = 12 + 6 = 18 m / sec . required time = ( 120 + 120 ) / 18 = 13 sec . answer : b"
Answer: B