A train crosses a bridge of length 800 m in 45 seconds and a lamp post on the bridge in 15 seconds. What is the length of the train in metres?
Choices
A) 375 m  B) 750 m  C) 400 m  D) 800 m  E) 300 m

Rationale: "let length of train = l case - 1 : distance = 800 + l ( while crossing the bridge ) time = 45 seconds i . e . speed = distance / time = ( 800 + l ) / 45 case - 2 : distance = l ( while passing the lamp post ) time = 15 seconds i . e . speed = distance / time = ( l ) / 15 but since speed has to be same in both cases so ( 800 + l ) / 45 = ( l ) / 15 i . e . 800 + l = 3 l i . e . 2 l = 800 i . e . l = 400 answer : option c"
Answer: C