A train crosses a bridge of length 150 m in 7.5 seconds and a lamp post on the bridge in 2.5 seconds. What is the length of the train in metres?	Answers:
A) 37.5 m B) 75 m C) 25 m D) 80 m E) 30 m

Rationale: let length of train = l case - 1 : distance = 150 + l ( while crossing the bridge ) time = 7.5 seconds i . e . speed = distance / time = ( 150 + l ) / 7.5 case - 2 : distance = l ( while passing the lamp post ) time = 2.5 seconds i . e . speed = distance / time = ( l ) / 2.5 but since speed has to be same in both cases so ( 150 + l ) / 7.50 = ( l ) / 2.5 i . e . 3 l = l + 150 i . e . 2 l = 150 i . e . l = 75 answer : option b
Answer: B