Answer:
The constant of variation, in this context, would be the time, because it's the ratio between the distance covered and the average speed.
So, to find each constant, we use the definition of a constant movement:

Motorcycle.


Using the constant relation, we have:

Bus.



Truck.



Bicycle.



There we have each ratio for each case. Overall, there's no a constant variation between times, because the bicycle is way to different from the motorcycle and truck time, that's why we take each ration as a constant for each case particularly.