A train crosses a bridge of length 2500 m in 120 seconds and a lamp post on the bridge in 30 seconds. What is the length of the train in metres ?	Select
A) 375 m	B) 750 / 3 m	C) 2500 / 3 m	D) 800 m	E) 300 m

Rationale: "let length of train = l case - 1 : distance = 2500 + l ( while crossing the bridge ) time = 120 seconds i . e . speed = distance / time = ( 2500 + l ) / 120 case - 2 : distance = l ( while passing the lamp post ) time = 30 seconds i . e . speed = distance / time = ( l ) / 30 but since speed has to be same in both cases so ( 2500 + l ) / 120 = ( l ) / 30 i . e . 4 l = 2500 + l i . e . 3 l = 2500 i . e . l = 2500 / 3 answer : option c"
Answer: C