A trained covered x km at 40 kmph and another 4x km at 20 kmph. Find the average speed of the train in covering the entire 3x km. Choose the correct choice from the following answers: A) 11.33 kmph B) 12.33 kmph C) 13.33 kmph D) 14.33 kmph E) 23.33 kmph

Rationale: "total time taken = x / 40 + 4 x / 20 hours = 9 x / 40 hours average speed = 3 x / ( 9 x / 40 ) = 13.33 kmph answer : c"
Answer: C