Calculation of Average Speed for the Whole Journey

The average speed for the whole journey is calculated as follows:

Step 1: Calculate the time taken for each distance travelled.

- For the first distance of 12 km at 3 km/hr, time taken = distance/speed = 12/3 = 4 hours
- For the second distance of 12 km at 4 km/hr, time taken = distance/speed = 12/4 = 3 hours
- For the third distance of 12 km at 6 km/hr, time taken = distance/speed = 12/6 = 2 hours

Step 2: Calculate the total distance travelled.

- Total distance travelled = 12 + 12 + 12 = 36 km

Step 3: Calculate the total time taken.

- Total time taken = 4 + 3 + 2 = 9 hours

Step 4: Calculate the average speed for the whole journey.

- Average speed = total distance/total time = 36/9 = 4 km/hr

Therefore, the man's average speed for the whole journey is 4 km/hr.

Explanation of the Calculation

The average speed is calculated by taking the total distance travelled and dividing it by the total time taken. In this case, the man travelled three different distances at different speeds. To calculate the time taken for each distance, we use the formula distance/speed. Once we have calculated the time taken for each distance, we add them up to find the total time taken for the whole journey. Similarly, we add up the distances to find the total distance travelled. Finally, we divide the total distance by the total time to find the average speed for the whole journey.

The average speed takes into account the different speeds at which the man travelled during the journey, and gives us an overall measure of his speed. It is important to note that the average speed is not necessarily the same as the speed at which the man travelled for any particular distance.