Thirty three because I'm just smart like that so you should believe me entirely.
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.
Goes +3
18, 21, 24, 27, 30
The slope is 4 or you can think of it as 4/1 if that helps. Remember slope intercept form is y=mx+b.