To find the average speed for the entire journey (both there and back), we need to use the formula for average speed, which is:

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

Step 1: Calculate the distance from A to B.

• The car travels at an average speed of 75 km/h for 4 hours, so: $\text{Distance from A to B} = 75 \, \text{km/h} \times 4 \, \text{hours} = 300 \, \text{km}$

Since the distance from A to B is 300 km, the return journey is also 300 km.

Step 2: Calculate the total distance for the round trip.

$\text{Total Distance} = 300 \, \text{km} + 300 \, \text{km} = 600 \, \text{km}$

Step 3: Calculate the total time for the round trip.

• The time to travel from A to B is 4 hours, and the time to return from B to A is 5 hours. $\text{Total Time} = 4 \, \text{hours} + 5 \, \text{hours} = 9 \, \text{hours}$

Step 4: Calculate the average speed for the round trip.

$\text{Average Speed} = \frac{600 \, \text{km}}{9 \, \text{hours}} = 66.67 \, \text{km/h}$

Final Answer:

The average speed for the entire journey is approximately 66.67 km/h.

Would you like more details or have any questions? Here are some related questions for further exploration:

1. How do you calculate the average speed for a single part of the journey?
2. What would the average speed be if the return journey took 6 hours instead of 5?
3. How does changing the speed for one part of the journey affect the overall average speed?
4. What is the formula to calculate average speed in general terms?
5. If the distance were different for each leg of the trip, how would that affect the calculation?

Tip: Average speed is not simply the mean of the two speeds; it is based on total distance and total time!