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

The correct answer is 80.

Step 1: Understand the given data
The journey of the car is divided into three consecutive quarters of an hour:
- Distance covered in the first quarter of an hour (t₁ = 1/4 h): d₁ = 20 km
- Distance covered in the second quarter of an hour (t₂ = 1/4 h): d₂ = 25 km
- Distance covered in the third quarter of an hour (t₃ = 1/4 h): d₃ = 15 km

Step 2: Calculate the total distance covered
The total distance (D) is the sum of the distances covered in each part of the journey:

$D=d_1+d_2+d_3$

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

Step 3: Calculate the total time taken
Each quarter of an hour is equal to 1/4 of an hour. The total time (T) for the three quarters is:

$T=\frac{1}{4}\text{h}+\frac{1}{4}\text{h}+\frac{1}{4}\text{h}=\frac{3}{4}\text{h}$

Step 4: Calculate the average speed
Average speed is defined as the total distance covered divided by the total time taken:

$\text{Average Speed}=\frac{\text{Total Distance}}{\text{Total Time}}$

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

Therefore, the average speed of the car over the entire journey is 80 km/h.