20. Find the average speed of the trip

Abdul, while driving to school, computes the average speed for his trip to be 21$\mathit{km}.h^{-1}$. On his return trip along the same route, there is less traffic and the average speed is 36$\mathit{km}.h^{-1}$. What is the average speed for Abdul’s trip?

(Enter your answer with two decimal places)

 

Answer:  $\mathit{km}.h^{-1}$