Abdul, while driving to school, computes the average speed for his trip to be 20 km/h. On his return trip along the same route, there is less traffic and the average speed is 30 km/h. What is the average speed for Abdul's trip?

Let us assume that Abdul is travelling x kilometer distance while going to school.
Distance = Speed x Time
1)When it's average speed is 20 km/hr
x = v1 × t1 = 20 × t1
2)When it's average speed is 30 km/hr
x = v2 × t2 = 30 × t2
V_average = total distance travelled / total time taken
V_average = x + x / (x/20) + (x/30) = 2x / (3x + 2x)/60 = 2x / (5x/60) = 120x / 5x = 24 km h⁻¹