To calculate the average speed of the truck during the whole journey in meters per second (m/s), we follow these steps:

Step 1: Total Distance Calculation

The truck's journey consists of two parts:

1. From $M$ to $N$: $\text{Distance from } M \text{ to } N = \text{Speed} \times \text{Time} = 60 \, \text{km/h} \times 30 \, \text{hours} = 1800 \, \text{km}.$

2. From $N$ to $M$: Since the truck returns on the same road, the distance is the same: $1800 \, \text{km}$.

The total distance for the whole journey: $\text{Total Distance} = 1800 \, \text{km} + 1800 \, \text{km} = 3600 \, \text{km}.$

Step 2: Total Time Calculation

1. Time taken from $M$ to $N$: $\text{Time from } M \text{ to } N = 30 \, \text{hours}.$

2. Time taken from $N$ to $M$: $\text{Time from } N \text{ to } M = \frac{\text{Distance}}{\text{Speed}} = \frac{1800 \, \text{km}}{40 \, \text{km/h}} = 45 \, \text{hours}.$

The total time for the whole journey: $\text{Total Time} = 30 \, \text{hours} + 45 \, \text{hours} = 75 \, \text{hours}.$

Step 3: Average Speed Calculation

The average speed of the whole journey is calculated using the formula: $\text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}}.$

In kilometers per hour: $\text{Average Speed} = \frac{3600 \, \text{km}}{75 \, \text{hours}} = 48 \, \text{km/h}.$

Convert this to meters per second ($\text{m/s}$): $\text{Average Speed in } \text{m/s} = 48 \, \text{km/h} \times \frac{1000 \, \text{m}}{1 \, \text{km}} \times \frac{1 \, \text{hour}}{3600 \, \text{seconds}} = \frac{48000}{3600} = 13.33 \, \text{m/s}.$

Final Answer:

The average speed of the truck during the whole journey is: $\boxed{13.33 \, \text{m/s}}$

---

Would you like further clarification or more details? Here are related questions to deepen your understanding:

1. What is the formula for converting km/h to m/s?
2. How does the time taken affect the average speed in cases of varying speeds?
3. What would the average speed be if the truck's return speed was 60 km/h instead of 40 km/h?
4. Can you explain why the average speed is not the arithmetic mean of 60 km/h and 40 km/h?
5. How would the calculation change if the truck traveled different distances for the two legs of the journey?

Tip: Always remember that the average speed for a round trip is determined by the total distance divided by the total time, not the arithmetic mean of the speeds.