A train traveling at 42 kms/ hr passes a cyclist going in the same direction in 9 secs. If the cyclist had been going in the opposite direction, the train would have passed him in 5 secs. Find the length of the train. Choose the correct choice from the following.
A) 75 meters B) 60 meters C) 90 meters D) 80 meters E) 70 meters

Rationale: let the length of the train be x meter and the speed of the cyclist is y m / s now when the train and the cyclist are in the same direction , the relative speed is : ( 35 / 3 - y ) m / s [ converting 42 km / h to m / s - 35 / 3 m / s ] now distance / relative speed = 9 seconds substituting the values , we get x + 9 y . . . . . . . . . . . . . . . . . . . . . ( 1 ) similarly calculate the relative speed when they are in the opposite direction the relative speed is ( 35 / 3 + y ) now distance / relative speed is equivalent to 5 seconds substituting the values , we get the equation 3 x - 15 y = 175 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( 2 ) solving ( 1 ) and ( 2 ) , we get x = 75 meters answer : a
Answer: A