Let one side distance = x km.

Time taken for forward trip at a speed of 20 km/h = Distance / Speed = x/20 h.

Time taken in return trip at a speed of 30 km/h = x/30 h.

Total time for the whole trip = / 20 + / 30 = 3+2 /60 = 5 60 h.

Total distance covered = 2x km.

We know, Average speed = Total distance ÷ Total time

= 2x ÷ (5x/60) = 24 kmh^{-1} .