Let the equal distance travelled is X

Then we know average velocity =Total distance/Total time taken

Here total distance will be= X+X+X

Total Distance=3X

Now time taken by each journey is distance/velocity

The There total time taken is sum of time in all three journey= X/60 +X/20+X/10

Total time =10X/60=X/6

Average Velocity of=3X/(X/6)=18kmph