Abdul, while driving to school, computes the average speed for his trip to be 20 km/hr. On his return trip along the same route, there is less traffic and the average speed is 40km/hr. What is the average sped for Abdul's trip?

You need to find the total distance and the total time. The total distance is two times the distance to school, 2d. s=d/t=>t=d/sWhere s is speed, d is distance and t is time. So the time taken to get to school is:d/s1and the time taken to get back is:d/s2so the total time is:d/s1+d/s2=d (1/s1+1/s2) So, as speed = distance/time, divide the total distance by the total time:2d/d (1/s1+1/s2) = 2 / (1/s1+1/s2) = 2 / (1/20+1/40) = 80/3≈26.7km/hr