**Average velocity** is defined as the total displacement divided by the total time taken. To calculate the average velocity of the bus, we need to determine the displacement and the time taken for both legs of the journey.

**Displacement from A to B:**

- The distance between town A and town B is the same for both legs of the journey.
- Let's assume this distance is 'd' km.

**Time taken from A to B:**

- The speed of the bus from town A to town B is given as 40 km/hr.
- Using the formula time = distance / speed, the time taken for this leg can be calculated as d / 40.

**Displacement from B to A:**

- Since the bus is returning from town B to town A, the displacement is in the opposite direction.
- Therefore, the displacement is -d km.

**Time taken from B to A:**

- The speed of the bus from town B to town A is given as 60 km/hr.
- Using the formula time = distance / speed, the time taken for this leg can be calculated as d / 60.

**Total displacement:**

- The total displacement of the bus is the sum of the displacements from A to B and from B to A.
- Therefore, the total displacement is d + (-d) = 0 km.

**Total time taken:**

- The total time taken by the bus is the sum of the times taken from A to B and from B to A.
- Therefore, the total time taken is d / 40 + d / 60.

**Average velocity:**

- Average velocity is defined as the total displacement divided by the total time taken.
- In this case, the total displacement is 0 km and the total time taken is d / 40 + d / 60.
- Dividing the total displacement by the total time taken gives us an average velocity of 0 km / (d / 40 + d / 60).
- Simplifying this expression further yields an average velocity of 0 km/hr.

Therefore, the average velocity of the bus is 0 km/hr. This means that the bus returns to its starting point in the same amount of time it takes to travel to the destination.

Average velocity will be zero because displacement is zero.