A car covers $20\text{km}$ in the first quarter of an hour, $25\text{km}$ in the second quarter of that hour and $15\text{km}$ in the third quarter. The average speed of the car (in $\text{km/h}$) over the entire journey is:

The average speed is defined as the total distance covered divided by the total time taken.
$$\text{Average Speed}=\frac{\text{Total Distance}}{\text{Total Time}}$$

Step 1: Calculate the total distance covered.

$$\text{Total Distance}=20\text{km}+25\text{km}+15\text{km}=60\text{km}$$

Step 2: Calculate the total time taken.
Each quarter of an hour represents $\frac{1}{4}$ of an hour. The car travels for three quarters in total:

$$\text{Total Time}=\frac{1}{4}+\frac{1}{4}+\frac{1}{4}=\frac{3}{4}\text{hour}$$

Step 3: Calculate the average speed.

$$\text{Average Speed}=\frac{60}{\frac{3}{4}}$$

$$\text{Average Speed}=60\times \frac{4}{3}=20\times 4=80\text{km/h}$$

Thus, the average speed over the entire journey is $80\text{km/h}$.