A car travels the first 1/3 of certain distance with a speed of 12km/hr the next 1/3 distance with a speed of 20 km/hr, and the last 1/3 distance with a speed of 60 km/hr. the average speed of the car for the whole journey is?  Choose the correct choice from the following choices.
A) 12km/hr  B) 20km/hr  C) 89km/hr  D) 52km/hr  E) 79km/hr
Let the whole distance travelled be x km   and the average speed of the car for he whole journey be y km/hr   then (x/3)/12+(x/3)/20+(x/3)/60=x/y   x/36+x/60+x/180=x/y   1/20y=1   y=20km/hr   Answer(20km/hr)
<result>B</result>
A car travels the first 1/3 of certain distance with a speed of 12km/hr the next 1/3 distance with a speed of 20 km/hr, and the last 1/3 distance with a speed of 60 km/hr. the average speed of the car for the whole journey is?