Cosine is positive in Quadrant IV.
Well, if it is increasing at a fixed rate of 2 mm/s, then the volume would still be increasing at a rate of 2 mm/s when the diameter is 80 mm, if I understand your question correctly.
You could 3•2 because your it two times =6 than it’s 2\6 make more smaller 1/3 it will be. Than you get .3333 repeating