A car travels the first 1/2 of certain distance with a speed of 15 km/hr the next 1/4 distance with a speed of 20 km/hr, and the last 1/4 distance with a speed of 60 km/hr. the average speed of the car for the whole journey is?
Choose one.
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/2)/15+(x/4)/20+(x/4)/60=x/y   x/30+x/80+x/240=x/y   1/20y=1   y=20km/hr   Answer(20km/hr)
<result>B</result>
A car travels the first 1/2 of certain distance with a speed of 15 km/hr the next 1/4 distance with a speed of 20 km/hr, and the last 1/4 distance with a speed of 60 km/hr. the average speed of the car for the whole journey is?