A car moving on a straight road covers one third of the distance with 20 km/hr and the rest with 60 km/hr. The average speed is

To find the average speed of the car, we can use the fundamental definition of average speed:
$Average\; Speed\; =\; Total\; Distance\; /\; Total\; Time$

Let the total distance of the journey be $d$ km.
The journey is divided into two parts:
1. One-third of the distance, which is $d\; /\; 3$ km, is covered at a speed of $v$₁ = 20 km/hr.
2. The remaining distance, which is $d\; -\; (d\; /\; 3)\; =\; 2d\; /\; 3$ km, is covered at a speed of $v$₂ = 60 km/hr.

Next, we calculate the time taken for each part of the journey:
Let $t$₁ be the time taken for the first part:
$t$₁ = Distance / Speed = (d / 3) / 20 = d / 60 hours

Let $t$₂ be the time taken for the second part:
$t$₂ = Distance / Speed = (2d / 3) / 60 = 2d / 180 = d / 90 hours

Now, we find the total time $T$ for the entire journey by adding $t$₁ and $t$₂:
$T\; =\; t$₁ + t₂ = (d / 60) + (d / 90)

To add these fractions, we find a common denominator, which is 180:
$T\; =\; (3d\; /\; 180)\; +\; (2d\; /\; 180)\; =\; 5d\; /\; 180\; =\; d\; /\; 36$ hours

Finally, we calculate the average speed:
$Average\; Speed\; =\; d\; /\; T\; =\; d\; /\; (d\; /\; 36)\; =\; 36$ km/hr

Thus, the average speed of the car is 36 km/hr.