An automobile travels from city $A$ to city $B$ and returns to city $A$ by the same route. The speed of the vehicle during the onward and return journeys were constant at $60km/h$ and $90 km/h$, respectively. What is the average speed in $km/h$ for the entire journey?
- $72\hspace{0.2cm} km/h$
- $73\hspace{0.2cm} km/h$
- $74\hspace{0.2cm} km/h$
- $75 \hspace{0.2cm}km/h$