**Average Speed of the Train**

To calculate the average speed of the train, we need to find the total distance traveled by the train and the total time taken for the entire journey.

**Step 1: Calculate the time taken for each leg of the journey**

First, let's calculate the time taken for the first leg of the journey, where the train travels a distance of 20 km at a speed of 60 km/h.

Time taken = Distance / Speed = 20 km / 60 km/h = 1/3 hours

Next, let's calculate the time taken for the second leg of the journey, where the train travels a distance of 40 km at a speed of 80 km/h.

Time taken = Distance / Speed = 40 km / 80 km/h = 1/2 hours

**Step 2: Calculate the total distance traveled**

The total distance traveled by the train is the sum of the distances covered in each leg of the journey.

Total distance = 20 km + 40 km = 60 km

**Step 3: Calculate the total time taken**

The total time taken by the train is the sum of the time taken for each leg of the journey.

Total time taken = 1/3 hours + 1/2 hours = 5/6 hours

**Step 4: Calculate the average speed**

Now, we can calculate the average speed of the train using the formula:

Average speed = Total distance / Total time taken

Average speed = 60 km / (5/6) hours = 72 km/h

Therefore, the average speed of the train is 72 km/h.

**Explanation**

Average speed is calculated by dividing the total distance traveled by the total time taken. In this case, the train traveled a distance of 20 km at a speed of 60 km/h, and a distance of 40 km at a speed of 80 km/h.

To find the total distance, we add the distances of each leg of the journey. To find the total time taken, we add the time taken for each leg of the journey.

Finally, we divide the total distance by the total time taken to find the average speed of the train. In this case, the average speed is found to be 72 km/h.