A participated in cycling contest and he drove the lap at the rate of 6kmph, 12kmph, 18kmph, 24kmph..What is his average speed?  Select.
A) 15 kmph  B) 18 kmph  C) 14.25 kmph  D) 11.52 kmph  E) 16 kmph

Rationale: = 4 x / ( x / 6 + x / 12 + x / 18 + x / 24 ) = 11.52 answer : d
Answer: D