A car travels first one-third of a certain distance with a speed of 10 km/hr

A car travels first one-third of a certain distance with a speed of 10 km/hr, the next one-third of the distance with the 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

  1. 18 km/hr
  2. 24 km/hr
  3. 30 km/hr
  4. 36 km/hr

Answer

Let the total distance be 3x km. Now, average speed is given by 

average speed = (total distance travelled) / (total time taken)

So, let times taken for the first, second and third halves of the journeys be t1, t2, and t3 respectively. Then, 

t1 = x/10

t2 = x/20

t3 = x/60

So, average speed = x / (t1 + t2 + t3)

= x / (x/10 + x/20 + x/60)

= 3/ ((6 + 3 + 1)/60)

= (3×60)/10

= 18 km/hr

The correct option is A.