Assume journey segments

Let the total distance be $3d$, meaning each one-third portion covers a distance of $d$.

$\text{Distance per segment} = d$

Calculate time for each segment

Compute the time taken for each of the three parts using the formula time equals distance over speed.

$t_1 = \frac{d}{20}, \quad t_2 = \frac{d}{30}, \quad t_3 = \frac{d}{60}$

Find total time

Add the individual times to find the total time taken for the journey.

$T = \frac{d}{20} + \frac{d}{30} + \frac{d}{60} = \frac{6d}{60} = \frac{d}{10}$

Calculate average speed

Divide the total distance by the total time to arrive at the final average speed.

$\text{Average Speed} = \frac{3d}{\frac{d}{10}} = 30 \text{ km/h}$