To find the average velocity, we need to calculate the total displacement and divide it by the total time taken.
The total displacement can be found by adding the individual displacements for each time interval. In the first hour, the body covers a distance of 60 m/s × 1 hour = 60 km. In the next two hours, the body covers a distance of 30 m/s × 2 hours = 60 km (since the velocity is constant during this time). Therefore, the total displacement is 60 km + 60 km = 120 km.
The total time taken is 1 hour + 2 hours = 3 hours.
Now, we can calculate the average velocity:
Average velocity = Total displacement / Total time taken = 120 km / 3 hours = 40 km/h
Therefore, the average velocity of the body is 40 km/h.