In a journey, the first one-third of the distance is covered at a speed of 20 km/hr, the second `1/3` at speed of 30 km/hr and the last `1/3` at the speed of 24 km/hr. The average speed in km/hr for the whole journey is
`75/3`
`73/3`
`74/3`
`72/3`
how is it possible???