Two trains of equal lengths take 8 sec and 15 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? Options:	A) 22	B) 12	C) 77	D) 10	E) 21

Rationale: "speed of the first train = 120 / 8 = 15 m / sec . speed of the second train = 120 / 15 = 8 m / sec . relative speed = 15 + 8 = 23 m / sec . required time = ( 120 + 120 ) / 23 = 10 sec . answer : d"
Answer: D