A man is standing on a railway bridge which is 180 m long. He finds that a train crosses the bridge in 20 seconds but himself in 8 seconds. Find the length of the train and its speed. Choose the correct answer
A) 10 m / sec  B) 15 m / sec  C) 18 m / sec  D) 19 m / sec  E) 25 m / sec

Rationale: if length of train is l and speed of train is s , then l / s = 8 ( l + 180 ) / s = 20 ( l + 180 ) / l = 20 / 8 = 5 / 2 2 l + 360 = 5 * l 3 * l = 360 l = 120 mtrs s = l / 8 = 120 / 8 = 15 m / sec answer : b
Answer: B