The correct option is B 18 km/h
Let total distance covered by the car be d
So, we have, average speed as
vavg=Total distanceTotal time
So, for 1/3rd distance, time taken is given by
t1=d3×10=d30 hr
for next 1/3rd distance, time taken is given by
t2=d3×20=d60 hr
Similarly, for next 1/3rd distance, time taken is given by
t3=d3×60=d180 hr
So, total time taken is t1+t2+t3=d30+d60+d180=d18
Thus, average speed is given by
vavg=dd18=18 km/h