A trained covered x km at 40 kmph and another 2x km at 20 kmph. Find the average speed of the train in covering the entire 5x km. Select:
A) 16
B) 18
C) 24
D) 40
E) 12

Rationale: "total time taken = x / 40 + 2 x / 20 hours = 5 x / 40 = x / 8 hours average speed = 5 x / ( x / 8 ) = 40 kmph answer : d"
Answer: D