1.  A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance with a speed of 20 km/hr, and the last one-third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is :
A. 18 km / hr
B. 24 km / hr
C. 30 km / hr
D. 36 km / hr

Answer: Option A

Explanation:

Let the whole distance travelled be x km and the average speed of the car for the whole journey by y km / hr.

Then, ( x/3)/10 + ( x+ 3)/20 + (x + 3)/60 = x/y

=> x/30 + x/60 + x/180 = x/y

=> 1y/18 = 1 <=> y = 18 km / hr