A car travels the first $180\text{km}$ at a speed of $20\text{km/h}$. It covers the next $210\text{km}$ at a speed of $35\text{km/h}$. Find the average speed of the car (in km/h).

The formula for average speed is:
$$\text{Average Speed}=\frac{\text{Total Distance}}{\text{Total Time}}$$
Total distance covered by the car:
$$\text{Total Distance}=180+210=390\text{km}$$
Time taken for the first part of the journey:
$$t_1=\frac{180}{20}=9\text{ hours}$$
Time taken for the second part of the journey:
$$t_2=\frac{210}{35}=6\text{ hours}$$
Total time taken:
$$\text{Total Time}=9+6=15\text{ hours}$$
Therefore, the average speed is:
$$\text{Average Speed}=\frac{390}{15}=26\text{km/h}$$