A car travels from A to B at a, speed of $20km{h}^{-1}$ and returns at a speed of $30km{h}^{-1}$. The average speed of the car for the whole journey is:

Let distance from A to B be x
Time for travel from A to B= x/20
Time for travel from B to A= x/30
Hence,speed for total distance 2x is: $\frac{2x}{\frac{x}{30}+\frac{x}{20}}=\frac{2x\times 60}{5x}$=24 km\hr