A train moves past a post and a platform 264 m long in 8 seconds and 20 seconds respectively. What is the speed of the train?  Choose one
A) 79.2 km / hr
B) 69 km / hr
C) 74 km / hr
D) 61 km / hr
E) none of these

Rationale: let x is the length of the train and v is the speed time taken to move the post = 8 s = > x / v = 8 = > x = 8 v - - - ( 1 ) time taken to cross the platform 264 m long = 20 s ( x + 264 ) / v = 20 = > x + 264 = 20 v - - - ( 2 ) substituting equation 1 in equation 2 , we get 8 v + 264 = 20 v = > v = 264 / 12 = 22 m / s = 22 × 36 / 10 km / hr = 79.2 km / hr answer : b
Answer: B