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 3x km.  Pick
A) 18 kmph B) 17 kmph C) 24 kmph D) 19 kmph E) 12 kmph

Rationale: "total time taken = x / 40 + 2 x / 20 hours = 5 x / 40 = x / 8 hours average speed = 3 x / ( x / 8 ) = 24 kmph answer : c"
Answer: C