A train speeds past a pole in 20 seconds and speeds past a platform 100 meters in length in 30 seconds. What is the length of the train?

Answer: Option D

Let the length of the train be x meters and its speed be y m/sec.
Then, $$\frac{\text{x}}{\text{y}}$$ = 20
⇒ y = $$\frac{\text{x}}{20}$$
∴ $$\frac{\text{x}+100}{30}$$  = $$\frac{\text{x}}{20}$$
⇒ 30x = 20x + 2000
⇒ 10x = 2000
⇒ x = 200 meters