To find the average speed of the car for the whole journey, we can use the concept of harmonic mean.

Let`s assume that the total distance is `d`. Since the car travels one-third of the distance at each speed, each one-third distance would be (d/3).

The time taken to cover the first one-third distance at 10 km/hr is (d/3) / 10 = d/30 hours.

The time taken to cover the next one-third distance at 20 km/hr is also (d/3) / 20 = d/60 hours.

The time taken to cover the last one-third distance at 60 km/hr is (d/3) / 60 = d/180 hours.

The total time taken for the entire journey is (d/30) + (d/60) + (d/180) = (6d + 3d + d)/180 = 10d/180 = d/18 hours.

Now, the average speed is calculated by dividing the total distance (d) by the total time taken (d/18):

Average speed = d / (d/18) = 18 km/hr.

Therefore, the average speed of the car for the whole journey is 18 km/hr.