A train speeds past a pole in 20 seconds and passes through a tunnel 500 m long in 40 seconds. Its length is :  Options:
A) 200 m
B) 400 m
C) 500 m
D) 700 m
E) none of the above

Rationale: sol . let the length of the train be x metres and its speed be y m / sec . then , x / y = 20 ⇒ y = x / 20 ∴ ( x + 500 ) / 40 = x / 20 ⇔ x = 500 m . answer c
Answer: C