A 300 m long train crosses a platform in 39 sec while it crosses a signal pole in 20 sec. What is the length of the platform?
Choose the correct choice from the following answers:
A) 389 m  B) 350 m  C) 285 m  D) 299 m  E) 219 m

Rationale: "speed = 300 / 20 = 15 m / sec . let the length of the platform be x meters . then , ( x + 300 ) / 39 = 15 = > x = 285 m . answer : c"
Answer: C