A car travells the first 1/2 of certain distance with a speed of 10km/hr the next 1/4 distance witha a speed of 20 km/hr, and the last 1/4 distance with a speed of 60 km/hr. the average speed of the carfor the whole journey is?	Choose the correct answer:
A) 12km/hr
B) 15km/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)/10+(x/4)/20+(x/4)/60=x/y   x/20+x/80+x/240=x/y   1/15y=1   y=15km/hr   Answer(15km/hr)
<result>B</result>
A car travells the first 1/2 of certain distance with a speed of 10km/hr the next 1/4 distance witha a speed of 20 km/hr, and the last 1/4 distance with a speed of 60 km/hr. the average speed of the carfor the whole journey is?