If the velocity of the object changes from 4 cm / sec to 16 cm / sec in 6 seconds, then at what average rate is the velocity changing?

Starting from rest, a ball rolls down a constant incline.  It requires 7.1 seconds to roll the 30 centimeter length of the incline.

An object which accelerates uniformly from rest will attain a final velocity which is double its average velocity.

In 10 seconds, a bicycle accelerating uniformly from rest covers a distance of 139 meters.  At what average rate is its velocity changing?